update soroban-rpc release documentation

[mollykarcher]

Platform maintains a document outlining the release process for soroban-rpc here. This document is somewhat outdated, and lacking in certain nuance/specificity in certain areas. For example:

• Special cases for docker-only image updates
• Detail around which quickstart images to update (and when)
• Detail around using unstable/testing versions of core
• Assumes there's always a corresponding go monorepo release
• etc etc etc

We should both update this document and remove the outward-facing documentation here. This leaks internal details as to how RPC is deployed and isn't needed to display in the public repo here.