Add documentation for SSE image upload config

Adds documentation for Static Site Editor config
file image_upload_path config entry.

Also adds explicit section headers and info
for different config files

doc/user/project/static_site_editor/index.md

@@ -66,10 +66,12 @@ easily [edit your content](#edit-content).
 1. To get started, create a new project from the [Static Site Editor - Middleman](https://gitlab.com/gitlab-org/project-templates/static-site-editor-middleman)
    template. You can either [fork it](../repository/forking_workflow.md#creating-a-fork)
    or [create a new project from a template](../../../gitlab-basics/create-project.md#built-in-templates).
-1. Edit the [`data/config.yml`](#configuration-files) configuration file
+1. Edit the [`data/config.yml`](#static-site-generator-configuration) configuration file
    to replace `<username>` and `<project-name>` with the proper values for
-   your project's path. This triggers a CI/CD pipeline to deploy your project
-   with GitLab Pages.
+   your project's path.
+1. (Optional) Edit the [`.gitlab/static-site-editor.yml`](#static-site-editor-configuration-file) file
+   to customize the behavior of the Static Site Editor.
+1. When you submit your changes, GitLab triggers a CI/CD pipeline to deploy your project with GitLab Pages.
 1. When the pipeline finishes, from your project's left-side menu, go to **Settings > Pages** to find the URL of your new website.
 1. Visit your website and look at the bottom-left corner of the screen to see the new **Edit this page** button.
 
@@ -179,6 +181,41 @@ yet. You can do so by editing the file locally, through the GitLab regular file
 
 ## Configuration files
 
+You can customize the behavior of a project which uses the Static Site Editor with
+the following configuration files:
+
+- The [`.gitlab/static-site-editor.yml`](#static-site-editor-configuration-file), which customizes the
+  behavior of the Static Site Editor.
+- [Static Site Generator configuration files](#static-site-generator-configuration),
+  such as `data/config.yml`, which configures the Static Site Generator itself.
+  It also controls the **Edit this page** button when the site is generated.
+
+### Static Site Editor configuration file
+
+> [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/4267) in GitLab 13.6.
+
+The `.gitlab/static-site-editor.yml` configuration file contains entries you can
+use to customize behavior of the Static Site Editor (SSE). If the file does not exist,
+default values which support a default Middleman project configuration are used.
+The [Static Site Editor - Middleman](https://gitlab.com/gitlab-org/project-templates/static-site-editor-middleman) project template generates a file pre-populated with these defaults.
+
+To customize the behavior of the SSE, edit `.gitlab/static-site-editor.yml`'s entries
+(described in the table below) according to what works best for your project (respecting YAML syntax).
+
+After the table, see an [example of the SSE configuration file](#gitlabstatic-site-editoryml-example).
+
+| Entry | GitLab version | Type | Default value | Description |
+|---|---|---|---|---|
+| `image_upload_path` | [13.6](https://gitlab.com/gitlab-org/gitlab/-/issues/216641) | String | `source/images` | Directory for images uploaded from the WYSIWYG editor. |
+
+#### `.gitlab/static-site-editor.yml` example
+
+```yaml
+image_upload_path: 'source/images' # Relative path to the project's root. Don't include leading or trailing slashes.
+```
+
+### Static Site Generator configuration
+
 The Static Site Editor uses Middleman's configuration file, `data/config.yml`
 to customize the behavior of the project itself and to control the **Edit this
 page** button, rendered through the file [`layout.erb`](https://gitlab.com/gitlab-org/project-templates/static-site-editor-middleman/-/blob/master/source/layouts/layout.erb).