Document `-replica` endpoints

[AnouckColson]

Where on docs.platform.sh should be changed?

• https://docs.upsun.com/add-services/mysql.html
• https://docs.upsun.com/add-services/postgresql.html
• https://docs.upsun.com/add-services/clickhouse.html
• https://docs.upsun.com/add-services/redis.html

What exactly should be updated?

For MariaDB/MySQL, Redis, ClickHouse, and PostgreSQL only.

Users can leverage the -replica endpoint when defining a relationship to a service, so that access is only granted to a replica instance instead of the actual db. It's also possible to restrict user permissions to read-only on that replica instance.

See Slack threads in Context field for further info and example configuration snippet.

Additional context

No response