Code References: Add GitLab integration guides

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](https://docs.gitlab.com/ci/variables/#define-a-cicd-variable-in-the-ui) to their GitLab project and one `include` to their `.gitlab-ci.yml`:

```yaml
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//...`)
- `FLAGSMITH_CODE_REFERENCES_API_KEY` — from _Organisation Settings > API Keys_, set as [masked](https://docs.gitlab.com/ci/variables/#define-a-cicd-variable-in-the-ui) 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 [`$CI_PROJECT_URL`](https://docs.gitlab.com/ci/variables/predefined_variables.html) for `repository_url` and [`$CI_COMMIT_SHA`](https://docs.gitlab.com/ci/variables/predefined_variables.html) for `revision`, and explain the mapping.

The simple config uses [`include:remote`](https://docs.gitlab.com/ci/yaml/#includeremote) with [`inputs:`](https://docs.gitlab.com/ci/yaml/#includeinputs) — the advanced config replaces that with an explicit job so the developer can change the [`image`](https://docs.gitlab.com/ci/yaml/#image), [`stage`](https://docs.gitlab.com/ci/yaml/#stages), or [`rules`](https://docs.gitlab.com/ci/yaml/#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)