website/docs: add GitLab social login source guide (#22764)

* website/docs: add GitLab social login source guide

* website/docs: clean up GitLab social login guide

Restructure the GitLab source guide so GitLab.com and self-hosted setup are handled in one flow.

Agent-thread: https://sdko.org/internal/threads/019e9445-5367-7973-8c2e-9a6be0cedfb3
A7k-product: product
A7k-product-repo: 4
Co-authored-by: Agent <agent@svc.sdko.net>

---------

Co-authored-by: Dominic R <dominic@goauthentik.io>
Co-authored-by: Agent <agent@svc.sdko.net>

website/docs/users-sources/sources/social-logins/gitlab/index.mdx

@@ -0,0 +1,70 @@
+---
+title: Log in with GitLab
+sidebar_label: GitLab
+tags:
+    - source
+    - gitlab
+---
+
+Allows users to authenticate using their GitLab credentials by configuring GitLab as a federated identity provider via OAuth 2.0.
+
+## Preparation
+
+The following placeholders are used in this guide:
+
+- `authentik.company` is the FQDN of the authentik installation.
+- `gitlab.company` is the FQDN of the GitLab installation. Use `gitlab.com` for GitLab SaaS.
+
+## GitLab configuration
+
+To integrate GitLab with authentik, you need to create an OAuth application in GitLab.
+
+1. Log in to GitLab and navigate to the application settings:
+    - For a user-owned application, select your avatar, click **Edit profile**, and then navigate to **Access** > **Applications**.
+    - For a group-owned application, navigate to the group and then click **Settings** > **Applications**.
+    - For an instance-wide application, navigate to **Admin** > **Applications**.
+2. Click **Add new application** or **New application** and set the following values:
+    - **Name**: `authentik`
+    - **Redirect URI**: `https://authentik.company/source/oauth/callback/gitlab/`
+    - **Scopes**: select all four of the following:
+        - `read_user`
+        - `openid`
+        - `profile`
+        - `email`
+
+3. Click **Save application** and take note of the **Application ID** and **Secret**. These values are required in the next section.
+
+## authentik configuration
+
+To support the integration of GitLab with authentik, you need to create a GitLab OAuth source in authentik.
+
+1. Log in to authentik as an administrator and open the authentik Admin interface.
+2. Navigate to **Directory** > **Federation and Social login**, click **New Source**, and then configure the following settings:
+    - **Select type**: select **GitLab OAuth Source** as the source type.
+    - **Create GitLab OAuth Source**: provide a name, a slug which must match the slug used in the GitLab **Redirect URI** field (e.g. `gitlab`), and set the following required configurations:
+        - Under **Protocol settings**:
+            - **Consumer key**: set the **Application ID** from GitLab.
+            - **Consumer secret**: set the **Secret** from GitLab.
+        - Under **URL settings** _(self-hosted GitLab instances only)_:
+            - **Authorization URL**: `https://gitlab.company/oauth/authorize`
+            - **Access token URL**: `https://gitlab.company/oauth/token`
+            - **Profile URL**: `https://gitlab.company/oauth/userinfo`
+            - **OIDC Well-known URL**: `https://gitlab.company/.well-known/openid-configuration`
+            - **OIDC JWKS URL**: `https://gitlab.company/oauth/discovery/keys`
+3. Click **Finish** to save your settings.
+
+:::info Display new source on login screen
+For instructions on how to display the new source on the authentik login page, refer to the [Add sources to default login page documentation](../../index.md#add-sources-to-default-login-page).
+:::
+
+:::info Embed new source in flow :ak-enterprise
+For instructions on embedding the new source within a flow, such as an authorization flow, refer to the [Source Stage documentation](../../../../../add-secure-apps/flows-stages/stages/source/).
+:::
+
+## Source property mappings
+
+Source property mappings allow you to modify or gather extra information from sources. See the [overview](../../property-mappings/index.md) for more information.
+
+## Resources
+
+- [GitLab Docs - Configure GitLab as an OAuth 2.0 authentication identity provider](https://docs.gitlab.com/integration/oauth_provider/)