Make updating handbook page for maturity model more convenient

Currently, to update the maturity model in the handbook, we have to do something like this:

1. Go to the metrics-catalog directory in the runbooks repo.
2. Run jsonnet -J ../libsonnet/ maturity.jsonnet.
3. Copy the output to data/service_maturity.yml in the website repo.
4. Create a branch and an MR against the website.

This is all automatable. Ideally we'd also add a comment to the top of the maturity results (in YAML), so that people know how to update it.

• Add a job to our recent monthly pipeline to trigger the downstream job in the runbook
• In the job in runbook, we implement a script to generate the service maturity manifest, and expose as an artifact
• The upstream job downloads the artifact, pulls the latest version from https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/data/service_maturity.yml
• If there is a difference, push the file to a new branch in handbook repo and draft a MR to update