Improve documentation for layers and protocol

docs/protocol/layers/message.md

docs/protocol/layers/overview.md

docs/protocol/serialization/features.md

docs/protocol/serialization/handshake_messages.md

docs/protocol/serialization/primitive_objects.md

docs/protocol/serialization/signalproxy_messages.md

docs/protocol/serialization/signalproxy_objects.md

mkdocs.yml