feat: improve and rework documentation and tooling

docs/protocol/handshake/messages.md

docs/protocol/layers/framing.md

docs/protocol/layers/message.md

docs/protocol/overview.md

docs/protocol/serialization.md

docs/protocol/serialization/features.md

docs/protocol/serialization/primitive_objects.md

docs/protocol/signalproxy/messages.md

docs/protocol/signalproxy/objects.md

docs/protocol/signalproxy/rpc.md

docs/usage/faq.md

docs/usage/upgrade_guide.md

mkdocs.yml

overrides/main.html

theme/main.html