Why

Closes #5152 and #5117

What's changed

• Move singular Postgres topic into a postgres/ subdir to accommodate the new read-only replicas topic
• In the parent topics: mention the ability to define/use replicas, and provide a link to the new topics

Where are changes

MariaDB: https://docs.upsun.com/add-services/mysql.html - new "read-only replication" topic
PostgreSQL: https://docs.upsun.com/add-services.html > PostgreSQL is now a parent topic, with one child "read-only replicas"

Updates are for:

• platform (sites/platform templates)
• upsun (sites/upsun templates)