Add migration for alerts

What does this MR do and why?

Migrates alert boxes to shortcode format.

This covers these alert types: note, warning, flag, disclaimer. We are deprecating the info alert type, and the details alert type is a separate issue.

The disclaimer alert has some special handling as we now populate that text from a template rather than having writers copy/paste it from the style guide.

See https://gitlab-org.gitlab.io/technical-writing-group/gitlab-docs-hugo/shortcodes/#alert-boxes for examples of the new format.

Closes #9 (closed).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md and checkout this branch, migrate-alert-boxes
2. Pull down fresh docs, but don't run migrations yet: REMOVE_BEFORE_CLONE=true go run cmd/gldocs/main.go clone (clone script is unchanged)
3. Run just the alert migration against the GitLab docs: go run cmd/gldocs/main.go migrate alerts ../gitlab/doc
   • You should see ~3 warnings about invalid disclaimer content.
4. Look at the diff. Alerts are now in the new format: git -C ../gitlab/doc diff
   • Indention should be unchanged.
5. Pull down fresh docs again and run the end-to-end clone + migrate: REMOVE_BEFORE_CLONE=true make clone-docs-projects
6. Run make view and visit a few pages with alert boxes, such as:
   • http://localhost:1313/integration/oauth_provider/#hashed-oauth-application-secrets
     • Compare to: https://docs.gitlab.com/ee/integration/oauth_provider/#hashed-oauth-application-secrets
   • http://localhost:1313/update/background_migrations/#automatic-batch-size-optimization
     • Compare to: https://docs.gitlab.com/ee/update/background_migrations/#automatic-batch-size-optimization
   • http://localhost:1313/ci/runners/new_creation_workflow/
     • Compare to: https://docs.gitlab.com/ee/ci/runners/new_creation_workflow/

• Alerts should display similarly to how they do on the Nanoc site.
• Disclaimers flagged as invalid content are not transformed (e.g, http://localhost:1313/integration/arkose/).

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.