Update format for development docs

Because any maintainer can merge
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/397788

Light levels of polish before merging.

Add a note about having  the "database" label

- Add quick start guide for postgres.ai web interface
- #database-lab was deprecated in favor of the web interface
- Removed references to getting explain plans from chatops

Prettifiers try to make the tables even, but that can sacrifice
legibility, as has happened here. No content changes.

Related epic: https://gitlab.com/groups/gitlab-com/-/epics/1940

In https://gitlab.com/gitlab-com/www-gitlab-com/-/merge_requests/104673,
we split the Enablement stage into Systems and Data Stores. This commit
changes the Database group to the Data Stores stage.