Merge branch '367854-timeline-events-preview-endpoint' into 'master'

Add a timeline event preview markdown endpoint

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/96540



Merged-by: Jan Provaznik <jprovaznik@gitlab.com>
Approved-by: Roy Zwambag <rzwambag@gitlab.com>
Approved-by: Jan Provaznik <jprovaznik@gitlab.com>
Co-authored-by: Vitali Tatarintev <vtatarintev@gitlab.com>