Adjust markdown linting guidelines

Move details of markdownlint mostly to the documentation
index for a SSoT, and link apropriately

doc/development/documentation/index.md

@@ -463,7 +463,7 @@ The current tests are:
    to [check locally](#previewing-the-changes-live) before pushing to GitLab by executing the command
    `bundle exec nanoc check internal_links` on your local
    [`gitlab-docs`](https://gitlab.com/gitlab-org/gitlab-docs) directory. In addition,
-   `docs-lint` also runs [markdownlint](styleguide.md#markdown-rules) to ensure the
+   `docs-lint` also runs [`markdownlint`](#markdownlint) to ensure the
    markdown is consistent across all documentation.
 1. [`ee_compat_check`](../automatic_ce_ee_merge.md#avoiding-ce-ee-merge-conflicts-beforehand) (runs on CE only):
    When you submit a merge request to GitLab Community Edition (CE),
@@ -488,7 +488,7 @@ help you catch common issues before raising merge requests for review of documen
 The following are some suggested linters you can install locally and sample configuration:
 
 - [`proselint`](#proselint)
-- [`markdownlint`](#markdownlint)
+- [`markdownlint`](#markdownlint), which is the same as the test run in [`docs-lint`](#testing)
 
 NOTE: **Note:**
 This list does not limit what other linters you can add to your local documentation writing toolchain.
@@ -534,76 +534,56 @@ A file with `proselint` configuration must be placed in a
 
 #### `markdownlint`
 
-`markdownlint` checks that certain rules ([example](https://github.com/DavidAnson/markdownlint/blob/master/README.md#rules--aliases))
-are followed for Markdown syntax. Our [Documentation Style Guide](styleguide.md) and
-[Markdown Guide](https://about.gitlab.com/handbook/product/technical-writing/markdown-guide/)
+[`markdownlint`](https://github.com/DavidAnson/markdownlint) checks that markdown
+syntax follows [certain rules](https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#rules),
+and is used by the [`docs-lint` test](#testing) with a [configuration file](#markdownlint-configuration).
+Our [Documentation Style Guide](styleguide.md#markdown) and [Markdown Guide](https://about.gitlab.com/handbook/product/technical-writing/markdown-guide/)
 elaborate on which choices must be made when selecting Markdown syntax for GitLab
-documentation. This tool helps catch deviations from those guidelines, and matches the
-tests run on the documentation by [`docs-lint`](#testing).
+documentation. This tool helps catch deviations from those guidelines.
 
 `markdownlint` can be used [on the command line](https://github.com/igorshubovych/markdownlint-cli#markdownlint-cli--),
 either on a single Markdown file or on all Markdown files in a project. For example, to run
 `markdownlint` on all documentation in the [`gitlab-ce` project](https://gitlab.com/gitlab-org/gitlab-ce),
-run the following commands from within the `gitlab-ce` project:
+run the following commands from within your `gitlab-ce` project root directory, which will
+automatically detect the [`.markdownlint.json`](#markdownlint-configuration) config
+file in the root of the project, and test all files in `/doc` and its subdirectories:
 
 ```sh
-cd doc
-markdownlint **/*.md
+markdownlint 'doc/**/*.md'
 ```
 
-`markdownlint` can also be run from within editors using plugins. For example, the following plugins
- are available:
+If you wish to use a different config file, use the `-c` flag:
+
+```sh
+markdownlint -c <config-file-name> 'doc/**/*.md'
+```
+
+`markdownlint` can also be run from within text editors using [plugins/extensions](https://github.com/DavidAnson/markdownlint#related),
+such as:
 
 - [Sublime Text](https://packagecontrol.io/packages/SublimeLinter-contrib-markdownlint)
 - [Visual Studio Code](https://marketplace.visualstudio.com/items?itemName=DavidAnson.vscode-markdownlint)
-- [Others](https://github.com/DavidAnson/markdownlint#related)
+- [Atom](https://atom.io/packages/linter-node-markdownlint)
 
-##### Sample `markdownlint` configuration
+It is best to use the [same configuration file](#markdownlint-configuration) as what
+is in use in the four repos that are the sources for <https://docs.gitlab.com>. Each
+plugin/extension has different requirements regarding the configuration file, which
+is explained in each editor's docs.
 
-The following sample `markdownlint` configuration modifies the available default rules to:
+##### `markdownlint` configuration
 
-- Adhere to the [Documentation Style Guide](styleguide.md).
-- Apply conventions found in the GitLab documentation.
-- Allow the flexibility of using some inline HTML.
+Each formatting issue that `markdownlint` checks has an associated
+[rule](https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#rules).
+These rules are configured in the `.markdownlint.json` files located in the root of
+four repos that are the sources for <https://docs.gitlab.com>:
 
-```json
-{
-  "default": true,
-  "header-style": { "style": "atx" },
-  "ul-style": { "style": "dash" },
-  "line-length": false,
-  "no-trailing-punctuation": false,
-  "ol-prefix": { "style": "one" },
-  "blanks-around-fences": true,
-  "no-inline-html": {
-    "allowed_elements": [
-      "table",
-      "tbody",
-      "tr",
-      "td",
-      "ul",
-      "ol",
-      "li",
-      "br",
-      "img",
-      "a",
-      "strong",
-      "i",
-      "div",
-      "b"
-    ]
-  },
-  "hr-style": { "style": "---" },
-  "code-block-style": { "style": "fenced" },
-  "fenced-code-language": false,
-  "no-duplicate-header": { "allow_different_nesting": true },
-  "commands-show-output": false
-}
-```
+- <https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.markdownlint.json>
+- <https://gitlab.com/gitlab-org/gitlab-runner/blob/master/.markdownlint.json>
+- <https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/.markdownlint.json>
+- <https://gitlab.com/charts/gitlab/blob/master/.markdownlint.json>
 
-For [`markdownlint`](https://github.com/DavidAnson/markdownlint/), this configuration must be
-placed in a [valid location](https://github.com/igorshubovych/markdownlint-cli#configuration). For
-example, `~/.markdownlintrc`.
+By default all rules are enabled, so the configuration file is used to disable unwanted
+rules, and also to configure optional parameters for enabled rules as needed.
 
 ## Danger Bot
 

doc/development/documentation/styleguide.md

@@ -110,18 +110,11 @@ Hard-coded HTML is valid, although it's discouraged to be used while we have `/h
 ### Markdown Rules
 
 GitLab ensures that the Markdown used across all documentation is consistent, as
-well as easy to review and maintain, by testing documentation changes with
-[Markdownlint (mdl)](https://github.com/markdownlint/markdownlint). This lint test
-checks many common problems with Markdown, and fails when any document has an issue
+well as easy to review and maintain, by [testing documentation changes](index.md#testing) with
+[`markdownlint`](index.md#markdownlint). This lint test fails when any document has an issue
 with Markdown formatting that may cause the page to render incorrectly within GitLab.
 It will also fail when a document is using non-standard Markdown (which may render
-correctly, but is not the current standard in GitLab documentation).
-
-Each formatting issue that mdl checks has an associated [rule](https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md),
-and the rules that are currently enabled for GitLab documentation are listed in the
-[`.mdlrc.style`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.mdlrc.style) file.
-Configuration options are set in the [`.mdlrc`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/.mdlrc.style)
-file.
+correctly, but is not the current standard for GitLab documentation).
 
 ## Structure
 
@@ -460,7 +453,7 @@ For other punctuation rules, please refer to the
   This is to ensure that no document with wrong heading is going
   live without an audit, thus preventing dead links and redirection issues when
   corrected.
-- Leave exactly one new line after a heading.
+- Leave exactly one blank line before and after a heading.
 - Do not use links in headings.
 - Add the corresponding [product badge](#product-badges) according to the tier the feature belongs.