Updated best practices for Postgres + minor code syntax polish

[michaelbarnes]

This PR introduces the following changes to the resources/production-readiness-guide.mdx page of the docs:

• A new numbered item to the overview at the top of the page.
• Minor MD code syntax fixes which bothered me.
• A new section at the bottom of the page which has the initial Postgres specific best practices. This is not the only best practices we'll include, but focuses specifically on max_slot_wal_keep_size and how it impacts the PowerSync Service.