Update location of 4 moved database pages

• Link to the merge request of the new pages: gitlab!84530 (merged)
  • doc/development/database/post_deployment_migrations.md
  • doc/development/database/deleting_migrations.md
  • doc/development/database/background_migrations.md
  • doc/development/database/avoiding_downtime_in_migrations.md
• Give the MR a descriptive title.
• Link to the merge request that introduced the new page.

Related to gitlab#357191 (closed)