Add maturity model to metrics catalog
This MR adds a maturity model definition for our services. The idea is that rather than do manual work (boo!) to periodically update this model, we can generate it from our existing service and metrics catalogs, which already have the data.
That output can then be used as a data file in the website repo, which then generates this page: https://about.gitlab.com/handbook/engineering/infrastructure/service-maturity-model/
This MR doesn't implement every item, just enough to get that handbook page into a reasonable state with reasonable enough levels. Issues to fill in the missing items are tracked in the epic gitlab-com/gl-infra&398 (closed).
Edited by Sean McGivern