Update docs to reflect new gem location

What does this MR do and why?

We should put new gems in https://gitlab.com/gitlab-org/ruby/gems/ to make it easier to discover what gems we actually maintain or fork.

This came out of a need to fork gRPC and protobuf temporarily and test our changes in CI to support the big endian architecture 390x for IBM Z mainframes:

1. https://github.com/protocolbuffers/upb/pull/435
2. https://github.com/grpc/grpc/pull/27881

Screenshots or screen recordings

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc: @gitlab-org/maintainers/rails-backend