Document existing protocol details and set up improvement process

While reworking the rust-peer to have a UI abstraction, I realized that the messages sent between universal connectivity peers are not documented anywhere other than in protobuf files. I think it is time to document the existing message formats (e.g. peer discovery, chat message, file publish, file request) and then also define a process for proposing improvements/changes and tracking the support status for each implementation.

I think we should set up a `specs` folder for holding the spec docs and ~also a `specs/ucip` folder for universal connectivity improvement proposal docs and tracking their status.~

Start by reviewing the existing protobuf definitions containing the peer-discovery, chat-message, file-publish, and file-request formats; the issue proposes placing the resulting specs under a specs folder. Document the current formats and define the improvement proposal process with implementation support status, ensuring the listed message types are covered.

Written by the indexing model from the issue text.