Add .rst suffix to documentation files so that github auto-formats them

docs/client-server/specification.rst

docs/client-server/urls.rst

docs/code_style.rst

docs/documentation_style.rst

docs/model/presence.rst

docs/model/profiles.rst

docs/model/room-join-workflow.rst

docs/model/rooms.rst

docs/model/third-party-id.rst

docs/protocol_examples.rst

docs/python_architecture.rst

docs/server-server/protocol-format.rst

docs/server-server/security-threat-model.rst

docs/server-server/specification.rst

docs/terminology.rst

docs/versioning.rst