Add documenation and CHANGELOG entries for Firestore integration
This gets the 30-integrate-firestore branch into a state where it can be merged to master.
The 17.0 major release is coming on May 16, 2024! This version brings many exciting improvements to GitLab, but also removes some deprecated features. We are introducing three breaking change windows during which we expect breaking changes to be deployed to GitLab.com. You can read more about it on our blogpost. The second breaking change window begins 2024-04-29 09:00 UTC and ends 2024-05-01 22:00 UTC.
This gets the 30-integrate-firestore branch into a state where it can be merged to master.