(#17945) Include more detailed documentation for PostgreSQL and SSL

This adds a detailed section on how to configure PostgreSQL SSL connectivity in
its own section.

This includes details on public and self-signed setups, using JKS and even
disabling validation if a user desires.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #406 from kbarber/ticket/master/17945-expand_puppetdb_pg_ssl_docs

Ticket/master/17945 expand puppetdb pg ssl docs