Move connection string details from config page to connection pool page

[steve-chavez]

The https://postgrest.org/en/stable/references/configuration.html page is mixed with connection string details.

This can be moved to https://postgrest.org/en/stable/references/connection_pool.html to make it clearer.

A "connection string" section that explains libpq env vars, formats(uri-based vs key value), etc.

---

Also check other details that are on https://postgrest.org/en/stable/references/configuration.html#db-uri.

[steve-chavez]

Configuration page is a bit simpler now https://postgrest.org/en/latest/references/configuration.html. I'll just link to config from the connection pool.