fix: allow ingest pipeline to be triggered manually

This patch allows ingest jobs to be run manually as triggered pipelines by setting GITLAB_INGEST_TRIGGER to either dev or prod.

We can then ingest docs for older gitlab versions by triggering the ingest pipeline against an older version of the docs by pointing GITLAB_DOCS_REPO_REF to an old tag on gitlab-org/gitlab.

This allows us to unblock #619 (closed) and consequently https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/1332.