Add info on how to self-host the docs site

doc/administration/docs_self_host.md

+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# How to self-host the docs site **(FREE SELF)**
+
+The following guide describes how to use a local instance of the docs site with
+a self-managed GitLab instance.
+
+## Run the docs site
+
+The easiest way to run the docs site locally it to pick up one of the existing
+Docker images that contain the HTML files.
+
+Pick the version that matches your GitLab version and run it, in the following
+examples 14.5.
+
+### Host the docs site using Docker
+
+The Docker images use a built-in webserver listening on port `4000`, so you need
+to expose that.
+
+In the server that you host GitLab, or any other server that your GitLab instance
+can talk to, you can use Docker to pull the docs site:
+
+```shell
+docker run -it --rm -p 4000:4000 registry.gitlab.com/gitlab-org/gitlab-docs:14.5
+```
+
+If you use [Docker compose](../install/docker.md#install-gitlab-using-docker-compose)
+to host your GitLab instance, add the following to `docker-compose.yaml`:
+
+```yaml
+version: '3.6'
+services:
+  docs:
+    image: registry.gitlab.com/gitlab-org/gitlab-docs:14.5
+    hostname: 'https://gitlab.example.com'
+    ports:
+      - '4000:4000'
+```
+
+### Host the docs site using GitLab Pages
+
+You can also host the docs site with GitLab Pages.
+
+Prerequisite:
+
+- The Pages site URL must not use a subfolder. Due to the nature of how the docs
+  site is pre-compiled, the CSS and JavaScript files are relative to the
+  main domain or subdomain. For example, URLs like `https://example.com/docs/`
+  are not supported.
+
+To host the docs site with GitLab Pages:
+
+1. [Create a new blank project](../user/project/working_with_projects.md#create-a-blank-project).
+1. Create a new or edit your existing `.gitlab-ci.yml` file and add the following
+   `pages` job. Make sure the version is the same as your GitLab installation:
+
+   ```yaml
+   image: registry.gitlab.com/gitlab-org/gitlab-docs:14.5
+   pages:
+     script:
+     - mkdir public
+     - cp -a /usr/share/nginx/html/* public/
+     artifacts:
+       paths:
+       - public
+   ```
+
+1. (Optional) Set the Pages domain name. Depending on the type of the Pages website,
+   you have two options:
+
+   | Type of website | [Default domain](../user/project/pages/getting_started_part_one.md#gitlab-pages-default-domain-names) | [Custom domain](../user/project/pages/custom_domains_ssl_tls_certification/index.md) |
+   | --------------- | -------------- | ------------- |
+   | [Project website](../user/project/pages/getting_started_part_one.md#project-website-examples) | Not supported | Supported |
+   | [User or group website](../user/project/pages/getting_started_part_one.md#user-and-group-website-examples) | Supported | Supported |
+
+### Host the docs site on your own webserver
+
+Since the docs site is static, you can grab the directory from the container
+(under `/usr/share/nginx/html`) and use your own web server to host
+it wherever you want. Replace `<destination>` with the directory where the
+docs will be copied to:
+
+```shell
+docker create -it --name gitlab-docs registry.gitlab.com/gitlab-org/gitlab-docs:14.5
+docker cp gitlab-docs:/usr/share/nginx/html <destination>
+docker rm -f gitlab-docs
+```
+
+## Redirect the `/help` links to the new docs page
+
+When the docs site is up and running:
+
+1. [Enable the help page redirects](../user/admin_area/settings/help_page.md#redirect-help-pages).
+   Use the Fully Qualified Domain Name as the docs URL. For example, if you
+   used the [Docker method](#host-the-docs-site-using-docker) , enter `http://0.0.0.0:4000`.
+   You don't need to append the version, it is detected automatically.
+1. Test that everything works by selecting the **Learn more** link on the page
+   you're on. Your GitLab version is automatically detected and appended to the docs URL
+   you set in the admin area. In this example, if your GitLab version is 14.5,
+   `https://<instance_url>/` becomes `http://0.0.0.0:4000/14.5/`.
+   The link inside GitLab link shows as
+   `<instance_url>/help/user/admin_area/settings/help_page#destination-requirements`,
+   but when you select it, you are redirected to
+   `http://0.0.0.0:4000/14.5/ee/user/admin_area/settings/help_page/#destination-requirements`.
+
+## Caveats
+
+- You need to host the docs site under a subdirectory matching your GitLab version,
+  in the example of this guide `14.5/`. The
+  [Docker images](https://gitlab.com/gitlab-org/gitlab-docs/container_registry/631635)
+  hosted by the Docs team provide this by default. We use a
+  [script](https://gitlab.com/gitlab-org/gitlab-docs/-/blob/2995d1378175803b22fb8806ba77adf63e79f32c/scripts/normalize-links.sh#L28-82)
+  to normalize the links and prefix them with the respective version.
+- The version dropdown will show more versions which do not exist and will lead
+  to 404 if selected.
+- The search results point to `docs.gitlab.com` and not the local docs.
+- When you use the Docker images to serve the docs site, the landing page redirects
+  by default to the respective version, for example `/14.5/`, so you don't
+  see the landing page as seen at <https://docs.gitlab.com>.