docs(auth): 📝 add gitlab idp env variables

apps/docs/docs/self-hosting/configuration.md

@@ -80,6 +80,23 @@ You can create your own GitHub OAuth app [here](https://github.com/settings/deve
 
 </p></details>
 
+<details><summary><h3>GitLab (Auth)</h3></summary>
+<p>
+
+Used for authenticating with GitLab.
+Follow the official GitLab guide for creating OAuth2 applications [here](https://docs.gitlab.com/ee/integration/oauth_provider.html).
+The Authorization callback URL should be `$NEXTAUTH_URL/api/auth/callback/gitlab`
+
+| Parameter                    | Default | Description                                                                          |
+| ---------------------------- | ------- | -------------------------------------------------------------------------------------|
+| NEXT_PUBLIC_GITLAB_CLIENT_ID | --      | Application client ID. Also used to check if it is enabled in the front-end          |
+| GITLAB_CLIENT_SECRET         | --      | Application secret                                                                   |
+| NEXT_PUBLIC_GITLAB_BASE_URL  | --      | Base URL of the GitLab instance, e.g. `https://gitlab.com`                           |
+| NEXT_PUBLIC_GITLAB_NAME      | --      | Name of the GitLab instance, used for the SSO Login Button, e.g. `GitLab`            |
+| GITLAB_REQUIRED_GROUPS       | --      | Comma-separated list of groups the user has to be a direct member of, e.g. `foo,bar` |
+
+</p></details>
+
 <details><summary><h3>Facebook (Auth)</h3></summary>
 <p>