internals: extract frame-based protocol docs to own document wireprotocol.txt is quite long and difficult to digest. The frame-based protocol is effectively a standalone concept (and could even be used outside of Mercurial). So this commit extracts its docs to a standalone file. The first few paragraphs were rewritten as part of the extraction. Sections headers were adjusted accordingly. Existing referalls in wireprotocol.txt were updated to refer to the new doc / concept, which I've started referring to as `hgrpc`. I'm on the fence as to whether to move the HTTP and SSH transport details to the new doc as well. For now, I'm leaving them in wireprotocol.txt. Differential Revision: https://phab.mercurial-scm.org/D4443