Clearly document the responsibilities of a database maintainer

The documentation on database reviews and the responsibilities of a database maintainer is very brief: https://about.gitlab.com/handbook/engineering/development/database/

Especially compared to the documentation on backend reviews: https://about.gitlab.com/handbook/engineering/workflow/code-review/#maintainer

It would be nice to have a complete list of the duties of a Database Maintainer.

• In the handbook : gitlab-com/www-gitlab-com!29650 (merged)
• In the development docs

cc @gl-database