docs: add cleanup stage for preview deployments (gitlab)

[devalog]

Summary

Updates the GitLab CI/CD guide to cover preview-link cleanup. The pipeline now deletes a merge
request's preview deployment once the MR merges into the default branch, so stale previews don't
linger.

Changes to fern/products/docs/pages/developer-tools/gitlab.mdx:

• Adds a cleanup_preview stage that looks up the merged MR for the commit, extracts its source
  branch, and runs fern docs preview delete --id "$SOURCE_BRANCH".
• Scopes preview generation per-branch via --id "$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME" --force, so
  each MR gets a stable preview URL that the cleanup step can target.
• Replaces hardcoded "main" branch rules with $CI_DEFAULT_BRANCH so the pipeline works for repos
  whose default branch isn't main.
• Installs jq in before_script (needed to parse the MR lookup response) and swaps grep -oP for
  grep -oE for portability.
• Updates the intro, pipeline description, and closing paragraph to mention per-branch previews and
  preview deletion on merge.

Review & Testing Checklist for Human

• Run the updated .gitlab-ci.yml against a real GitLab project: open an MR, confirm a preview
  link is posted, merge it, and confirm the cleanup_preview job deletes the preview.
• Confirm the jq parse in cleanup_preview handles the "direct push to main" case
  gracefully (job should exit 0, not fail).

[github-actions]

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Here are the markdown pages you've updated:

• learn/docs/changelog/2026/4/22
• learn/docs/developer-tools/git-lab