Merge alpha_database into database

What does this MR do?

Moves the PG HA documentation out of its alpha home, and instead into docs/high_availability/database.md proper.

Are there points in the code the reviewer needs to double check?

What should we do with the old section (discussion started)

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/build/team-tasks/issues/62 As !3449 (merged) is not merged I expect a little in-flight merge conflict, depending on whichever lands first