Clean up RPC docs filesystem

- Move the existing RPC specs all into one /protocol/rpc directory. E.g. the supervisor RPC is already specified.
- Add references to the docs we moved these out of.
- Create a document for each RPC namespace. E.g. rpc-admin.md, rpc-optimism.md, rpc-engine.md, rpc-interop.md, rpc-supervisor.md, rpc-opp2p.md, etc.