Add new page for database load balancing

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!117382 (merged) adds https://docs.gitlab.com/ee/development/database/load_balancing.html
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Note to reviewer

Scrambling to get this one in under the deadline. It's super late on the 14th. Add development doc on database load balancing (gitlab!117382 - merged) should merge in a couple more minutes, which should make it a relatively safe bet for the %15.11 cutoff. I know I can merge this myself, but I'd really rather not.

Also, I'm the release person for this month and I happen to know I haven't cut the release branch yet. 😸