Add handbook page on Database Reviewer Onboarding

We should document the process of becoming a database reviewer, and provide guides on things to read, etc:

1. Summary of work done by database reviewers, process followed, etc
2. How to apply for becoming a database reviewer
3. Additional info:
   • database-labs
   • database office hours
   • It's a team game: (at)gl-database and slack channels where we reach out for help when we are uncertain on what action to take
4. Resources to read

   1. Links to our guides

   2. Links that explain EXPLAIN (pun intended)

      Explaining the unexplainable series in depesz is pretty nice

5. Examples of some interesting reviews to check
   1. A Migration review that asks for everything required
   2. A post migration one
   3. At least one background migration review
   4. A couple reviews on MRs where queries are optimized (one including namespaces)
   5. A review of a finder
6. Why using the 🔴 is a good thing if you are feeling overwhelmed
7. How to level up and gain more exp and loot drops by becoming a trainee database maintainer.