Add more protocol documentation

docs/assets/images/message_init.png

docs/assets/images/message_setup.png

docs/protocol/layers/framing.md

docs/protocol/layers/message.md

docs/protocol/serialization/compound.md

docs/protocol/serialization/features.md

docs/protocol/serialization/handshake.md

docs/protocol/serialization/primitive.md

mkdocs.yml