Update GCP billing alerts section and howto

Part of https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-product-factory/-/issues/123

Refactor the GCP billing section to follow the Diataxis structure. Also, added a how to guide for dealing with GCP billing alerts.

Failed pipeline doesn't relate to any of my changes by the looks of it so should be ignored 😸