RDSC-4040 add note for RDS proxy

[ZdravkoDonev-redis]

Currently we've documented that you should use RDS Proxy for all source databases, but that is not the case for the RDS Proxy. We have an initiative to try and improve this, but for the time being we must document the workaround for that database.
So here it is.

[jit-ci]

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[github-actions]

Staging links:
https://redis.io/docs/staging/RDSC-4040-add-note-for-RDS-proxy/operate/rc/databases/rdi/setup/

[Copilot]

Pull Request Overview

This PR adds documentation clarifying that RDS Proxy is not compatible with PostgreSQL databases and provides alternative setup instructions. The changes address a gap in the current documentation which incorrectly suggested using RDS Proxy for all source databases.

• Added warning about RDS Proxy incompatibility with PostgreSQL databases
• Provided alternative approaches for PostgreSQL in test and production environments
• Updated setup instructions to differentiate between MySQL/SQL Server and PostgreSQL workflows

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