docs(GitLab): Add integration documentation for GitLab.com and self-hosted

docs/docs/third-party-integrations/project-management/gitlab.md

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+---
+title: GitLab Integration
+description: Link GitLab issues and merge requests to Flagsmith feature flags
+sidebar_label: GitLab
+hide_title: true
+---
+
+<img src="/img/integrations/gitlab/gitlab-logo.svg" alt="GitLab Logo" width="30%" />
+
+Link GitLab issues and merge requests to your Flagsmith feature flags. When a
+flag changes state, Flagsmith posts a comment to the linked issue or MR showing
+the flag's current state across all environments.
+
+## Setup
+
+The integration supports
+[personal access tokens](https://docs.gitlab.com/user/profile/personal_access_tokens/),
+[group access tokens](https://docs.gitlab.com/user/group/settings/group_access_tokens/),
+and [project access tokens](https://docs.gitlab.com/user/project/settings/project_access_tokens/).
+All require the `api` scope. The token type determines which projects are
+accessible in Flagsmith.
+
+1. **In GitLab**
+   1. Create an access token with the `api` scope.
+   1. Copy the token — you will not see it again.
+1. **In Flagsmith**
+   1. Go to Integrations > **GitLab** > Add Integration.
+   1. Set the **GitLab Instance URL** to your instance
+      (e.g. `https://gitlab.example.com` or `https://gitlab.com`).
+   1. Paste the access token.
+   1. Click "Save". ✅
+
+:::tip
+
+Remember to rotate your GitLab access token before it expires.
+
+:::
+
+## Linking issues and merge requests to feature flags
+
+1. Open a feature flag and go to the **Link** tab.
+1. Select a GitLab project.
+1. Choose **Issue** or **Merge Request**.
+1. Search and select the item you want to link.
+
+Flagsmith will post a comment to the linked issue or MR with the flag's current
+state across all environments. When the flag state changes, a new comment is
+posted automatically.
+
+A **Flagsmith Flag** label is added to linked issues and merge requests so your
+team can filter for them in GitLab.
+
+## Automatic state sync
+
+When a linked issue or merge request changes state in GitLab — closed, merged,
+or reopened — Flagsmith automatically updates the linked feature flag's tags to
+reflect the current state. This is powered by webhooks that Flagsmith registers
+on your GitLab projects automatically.
+
+## Removing the integration
+
+Go to Integrations > GitLab > Manage Integration > Delete Integration.