Add some DB-related pages to nav

This MR adds some database development-related pages to the docs nav.

Still to be added: pages linked under:

• https://docs.gitlab.com/ee/development/database/#best-practices
• https://docs.gitlab.com/ee/development/database/#miscellaneous

cc @sselhorn because I remember you asked to be pinged on (significant?) nav updates.

Related to #1051