docs: split and expand backend docs (#3350)

website/content/docs/gitlab-backend.md

@@ -0,0 +1,58 @@
+---
+title: GitLab
+weight: 20
+group: backends
+---
+
+For repositories stored on GitLab, the `gitlab` backend allows CMS users to log in directly with their GitLab account. Note that all users must have push access to your content repository for this to work.
+
+The GitLab API allows for two types of OAuth2 flows:
+
+* [Web Application Flow](https://docs.gitlab.com/ce/api/oauth2.html#web-application-flow), which works much like the GitHub OAuth flow described above.
+* [Implicit Grant](https://docs.gitlab.com/ce/api/oauth2.html#implicit-grant-flow), which operates _without_ the need for an authentication server.
+
+## Web Application Flow with Netlify
+
+When using GitLab's Web Application Flow for authentication, you can use Netlify to handle the server-side authentication requests.
+
+To enable it:
+
+1. Follow the [GitLab docs](https://docs.gitlab.com/ee/integration/oauth_provider.html#adding-an-application-through-the-profile) to add your Netlify CMS instance as an OAuth application. For the **Redirect URI**, enter `https://api.netlify.com/auth/done`, and check the box for `api` scope.
+2. Follow the [Netlify docs](https://www.netlify.com/docs/authentication-providers/#using-an-authentication-provider) to add your new GitLab Application ID and Secret to your Netlify site dashboard.
+3. In your repository, add the following lines to your Netlify CMS `config.yml` file:
+
+```yaml
+backend:
+  name: gitlab
+  repo: owner-name/repo-name # Path to your GitLab repository
+```
+
+## Client-Side Implicit Grant (GitLab)
+
+With GitLab's Implicit Grant, users can authenticate with GitLab directly from the client. To do this:
+
+1. Follow the [GitLab docs](https://docs.gitlab.com/ee/integration/oauth_provider.html#adding-an-application-through-the-profile) to add your Netlify CMS instance as an OAuth application. For the **Redirect URI**, enter the address where you access Netlify CMS, for example, `https://www.mysite.com/admin/`. For scope, select `api`.
+2. GitLab gives you an **Application ID**. Copy this ID and enter it in your Netlify CMS `config.yml` file, along with the following settings:
+
+    ```yaml
+    backend:
+      name: gitlab
+      repo: owner-name/repo-name # Path to your GitLab repository
+      auth_type: implicit # Required for implicit grant
+      app_id: your-app-id # Application ID from your GitLab settings
+    ```
+
+    You can also use Implicit Grant with a self-hosted GitLab instance. This requires adding `api_root`, `base_url`, and `auth_endpoint` fields:
+
+    ```yaml
+    backend:
+      name: gitlab
+      repo: owner-name/repo-name # Path to your GitLab repository
+      auth_type: implicit # Required for implicit grant
+      app_id: your-app-id # Application ID from your GitLab settings
+      api_root: https://my-hosted-gitlab-instance.com/api/v4
+      base_url: https://my-hosted-gitlab-instance.com
+      auth_endpoint: oauth/authorize
+    ```
+
+**Note:** In both cases, GitLab also provides you with a client secret. You should _never_ store this in your repo or reveal it in the client.