Rewrite the docs

[Archmonger]

Summary

Live preview: https://reactive-python.github.io/reactpy/

This PR is a rewrite of our documentation to imitate React's documentation. Additionally, this switches us from sphinx to mkdocs as a documentation tool.

Sections titles with a 🚧 symbol are still undeveloped.

Issues

• fix Add Analytics to reactpy.dev #1037
• fix Outdated GIF in Docs #1034
• fix Unexpected offset in sample application #923
• fix Rewrite the docs #863
• fix Improve use_context docs #845
• fix Use snake_case for components #779
• fix Add Example of Bug Due to Mutation in "Dangers of Mutability" #568
• fix Add docs section for Common utils (e.g. the stuff in reactpy.widgets) #349
• fix Document the importSource key in the VDOM spec #304
• fix Document Test Tooling #264

Potentially will cover

• Follow NEP 29 (Drop Python 3.7 and 3.8) #977
• Expose and document the VDOM event handler interface #878
• Embedding ReactPy via Template Tags #653
• Improve JavaScript components docs #647
• Document How to Set Up a Development Environment with Docker #597