KR Database Team - Improve documentation and add demos to increase engineering knowledge of PostgreSQL => 50%

Title is self describing

• Move all database guides under the database directory - gitlab-org/gitlab#218664 (closed)
• Document situations when jsonb is appropriate to use (and when it's not) - gitlab-org/gitlab#220986
• Document design decisions for partitioning audit events - gitlab-org/gitlab#217489 (closed)
• Developer documentation for migration helpers to partition existing tables - gitlab-org/gitlab#219293 (closed)
• Document how to get access to GitLab.com database - !59801 (merged)
• Developer documentation for migration helpers to partition existing tables - gitlab-org/gitlab#219293 (closed)