Add development guidelines for wikis

Adds a new page in `doc/development`, along with some tweaks to our
Vale grammar checker.

doc/.vale/gitlab/Acronyms.yml

@@ -66,6 +66,7 @@ exceptions:
   - POST
   - PUT
   - RAM
+  - REST
   - RPC
   - RSA
   - RSS

doc/.vale/gitlab/spelling-exceptions.txt

@@ -167,6 +167,7 @@ Gitleaks
 Gitter
 globals
 Gmail
+Gollum
 Google
 Gosec
 Gradle

doc/development/README.md

@@ -116,6 +116,7 @@ Complementary reads:
 - [Code Intelligence](code_intelligence/index.md)
 - [Approval Rules](approval_rules.md)
 - [Feature categorization](feature_categorization/index.md)
+- [Wikis development guide](wikis.md)
 
 ## Performance guides
 

doc/development/wikis.md

+---
+type: reference, dev
+stage: none
+group: Development
+info: "See the Technical Writers assigned to Development Guidelines: https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments-to-development-guidelines"
+description: "GitLab's development guidelines for Wikis"
+---
+
+# Wikis development guide
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/227027) in GitLab 13.5.
+
+## Overview
+
+The wiki functionality in GitLab is based on [Gollum 4.x](https://github.com/gollum/gollum/),
+which is used in [Gitaly's](gitaly.md) Ruby service and accessed from the Rails app through Gitaly RPC calls.
+
+Wikis use Git repositories as storage backend, and can be accessed through:
+
+- The [Web UI](../user/project/wiki/index.md)
+- The [REST API](../api/wikis.md)
+- [Git itself](../user/project/wiki/#adding-and-editing-wiki-pages-locally)
+
+[Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2214) in GitLab 13.5, wikis are also available
+for groups, in addition to projects.
+
+## Involved Gems
+
+Some notable gems that are used for wikis are:
+
+| Component     | Description                                    | Gem name                       | GitLab project                                                                                          | Upstream project                                                    |
+|:--------------|:-----------------------------------------------|:-------------------------------|:--------------------------------------------------------------------------------------------------------|:--------------------------------------------------------------------|
+| `gitlab`      | Markup renderer, depends on various other gems | `gitlab-markup`                | [`gitlab-org/gitlab-markup`](https://gitlab.com/gitlab-org/gitlab-markup)                               | [`github/markup`](https://github.com/github/markup)                 |
+| `gitaly-ruby` | Main Gollum library                            | `gitlab-gollum-lib`            | [`gitlab-org/gollum-lib`](https://gitlab.com/gitlab-org/gollum-lib)                                     | [`gollum/gollum-lib`](https://github.com/gollum/gollum-lib)         |
+|               | Gollum Git adapter for Rugged                  | `gitlab-gollum-rugged_adapter` | [`gitlab-org/gitlab-gollum-rugged_adapter`](https://gitlab.com/gitlab-org/gitlab-gollum-rugged_adapter) | [`gollum/rugged_adapter`](https://github.com/gollum/rugged_adapter) |
+|               | Rugged (also used in Gitaly itself)            | `rugged`                       | -                                                                                                       | [`libgit2/rugged`](https://github.com/libgit2/rugged)               |
+
+### Notes on Gollum
+
+We only use Gollum as a storage abstraction layer, to handle the mapping between wiki page slugs and files in the repository.
+
+When rendering wiki pages, we don't use Gollum at all and instead go through a
+[custom Banzai pipeline](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/banzai/pipeline/wiki_pipeline.rb).
+This adds some [wiki-specific markup](../user/markdown.md#wiki-specific-markdown), such as Gollum's `[[link]]` syntax.
+
+Since we do not make use of most of Gollum's features, we plan to move away from it entirely at some point.
+[See this epic](https://gitlab.com/groups/gitlab-org/-/epics/2381) for reference.
+
+## Model classes
+
+The `Wiki` class is the main abstraction around a wiki repository, it needs to be initialized
+with a container which can be either a `Project` or `Group`:
+
+```mermaid
+classDiagram
+  Wiki --> ProjectWiki
+  Wiki --> GroupWiki
+
+  class Wiki {
+    #container
+    #repository
+  }
+
+  class ProjectWiki {
+    #project → #container
+  }
+
+  class GroupWiki {
+    #group → #container
+  }
+```
+
+Some models wrap similar classes from Gitaly and Gollum:
+
+| Rails Model | Gitaly Class                                            | Gollum         |
+|:------------|:--------------------------------------------------------|:---------------|
+| `Wiki`      | `Gitlab::Git::Wiki`                                     | `Gollum::Wiki` |
+| `WikiPage`  | `Gitlab::Git::WikiPage`, `Gitlab::Git::WikiPageVersion` | `Gollum::Page` |
+|             | `Gitlab::Git::WikiFile`                                 | `Gollum::File` |
+
+Only some data is persisted in the database:
+
+| Model                 | Description                              |
+|:----------------------|:-----------------------------------------|
+| `WikiPage::Meta`      | Metadata for wiki pages                  |
+| `WikiPage::Slug`      | Current and previous slugs of wiki pages |
+| `ProjectRepository`   | Gitaly storage data for project wikis    |
+| `GroupWikiRepository` | Gitaly storage data for group wikis      |
+
+## Attachments
+
+The web UI uploads attachments through the REST API, which stores the files as commits in the wiki repository.
+
+Prior to GitLab 11.3 attachments were stored outside of the repository, [see this issue](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/33475).