Code References: Add GitLab integration guides

Open docs/docs/managing-flags/code-references.md and compare its existing GitHub section, including the info box and related links. Review the GitLab template referenced by the issue before documenting both configurations, then verify that the simple example is copy-pasteable, the advanced job exposes image, stage, and rules, and the self-hosted URL override is covered.

Context: https://github.com/Flagsmith/flagsmith/issues/7000

The Code References docs page only covers GitHub. We need a GitLab section so a developer can set this up in under five minutes.

Scope

Add an "Integrate with GitLab" section to docs/docs/managing-flags/code-references.md, mirroring the existing GitHub section (simple + advanced). Update the info box (lines 17–21) that says GitHub-only.

Simple configuration

The developer adds two CI/CD variables to their GitLab project and one include to their .gitlab-ci.yml:

include:
  - remote: 'https://raw.githubusercontent.com/Flagsmith/ci/v1.0.0/templates/gitlab-ci.yml'
    inputs:
      flagsmith_project_id: "$FLAGSMITH_PROJECT_ID"

The two variables:

• FLAGSMITH_PROJECT_ID — from the Flagsmith dashboard URL (/project/<id>/...)
• FLAGSMITH_CODE_REFERENCES_API_KEY — from Organisation Settings > API Keys, set as masked so it doesn't leak in job logs

Optionally set FLAGSMITH_ADMIN_API_URL for self-hosted Flagsmith instances (defaults to https://api.flagsmith.com).

Advanced configuration

For teams that need to customise the pipeline (different stage, extra rules, multiple repos), show the full job definition that the template abstracts away. Use $CI_PROJECT_URL for repository_url and $CI_COMMIT_SHA for revision, and explain the mapping.

The simple config uses include:remote with inputs: — the advanced config replaces that with an explicit job so the developer can change the image, stage, or rules.

Acceptance criteria

• Simple config is copy-pasteable with zero code modifications
• Advanced config shows the full job for customisation
• Self-hosted Flagsmith URL covered as optional override
• Links to same related pages as the GitHub section (Admin API Authentication, Flag Lifecycle)