Docs: Add GitLab integration guide for Code References

[emyller]

Context: #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)