Docs - added Postgres require SSL section

Add documentation on how to require SSL/TLS connections for Omnibus/external Postgres databases.

I've added the documentation to both Omnibus/external Postgres sections to make this clear to users that it is supported on both.

Related issues

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.