doc: update the docs for the reference architectures and ensure that the docs stay up-to-date

This updates the README.md with more up-to-date information on the SLIs that we monitor for the GET-Hybrid reference architecture.

Review the markdown changes here: https://gitlab.com/gitlab-com/runbooks/-/blob/doc-improve-reference-architecture-readme-docs/reference-architectures/get-hybrid/README.md

Additionally, I've automated the update of the README, so going forward it's less likely that this documentation will go out-of-date again.