Add Documentation hosted on ReadTheDocs

[bollwyvl]

To document what is already available with the 0.5.1 release that works solidly with JupyterLab 3.0.x, we should:

Checklist

• [x] :unlock: register jupyter-videochat RTD project
  • i've already squatted this, and can add as co-owner whoever has perms to...
• [x] :lock: enable webhook on repo
• [x] :unlock: :envelope_with_arrow: request the mamba feature flag
  • because we'll need an up-to-date nodejs soon enough
• [ ] probably in one PR...
  • [ ] add a .readthedocs.yml
  • [ ] add a docs/environment.yml
  • [ ] myst-nb
  • [ ] pydata-sphinx-theme
  • [ ] sphinx-jsonschema
  • [ ] docs/index.md that re-uses README.md and CHANGELOG.md
  • [ ] docs/dev-guide.md that re-uses CONTRIBUTING.md
  • [ ] docs/conf.py to build the site