Update links to PostgreSQL docs, point to version 11

What does this MR do?

This MR updates all links in omnibus docs to point to the PosgreSQL 11 pages.

Related issues

Closes #5098 (closed).

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.