Clarify Postgres RDS source docs

[jpepin]

Motivation

Line-specific feedback and tweaks to the Postgres RDS source setup guide, as well as some general feedback.
This isn't mergeable as-is since there is a lot of meta-commentary.
This aligns strongly with MaterializeInc/database-issues#5640

Other Notes

• The sidebar Connect Sources section of the docs links right to the Postgres CDC page, skipping the top level postrgres source page which has most of the important setup detail. This is just a bit confusing when getting started.
• We should add a new Troubleshooting section for Connections, or include more detail on the Connection creation page for things like SHOW CREATE CONNECTION. I created a malformed connection the first time around and it was difficult to discover how to see it and then how to fix it (it has to be dropped and recreated). ALTER CONNECTION doesn't do what it looks like it does!
• It's not immediately obvious which doc sections apply to the Postgres database and which sections apply to Materialize, if you're not already familiar with both.