Add documentation for using Patroni with Omnibus

What does this MR do?

Closes omnibus-gitlab#5116 (closed) Also addresses omnibus-gitlab#5114 (closed)

This MR contains documentation for using Patroni for HA database deployment. Patroni is scheduled to be shipped with Omnibus in 13.1 release.

Screenshots

NA

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

NA