Merge branch 'chore/backfill-embeddings-locally' into 'master'

Add instructions and a helper to generate issue embeddings locally

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157645



Merged-by: John Mason <9717668-johnmason@users.noreply.gitlab.com>
Approved-by: John Mason <9717668-johnmason@users.noreply.gitlab.com>
Reviewed-by: Dmitry Gruzd <dgruzd@gitlab.com>
Co-authored-by: maddievn <mvanniekerk@gitlab.com>