Add docs/demo with ReadTheDocs

[bollwyvl]

It would be nice to use ReadTheDocs to get a documentation site up. This could include some generated API docs (especially for the plugin API) for the typescript side of the house, as well as provide more concrete examples of any built-in (or at least monorepo'd) plugins.

With a bit more work, this could also use jupyterlite (or jupyterlite-sphinx and get per-PR-push previews deployed.

Tasks

• baseline
  • in RTD web ui
    • reserve appropriate site name, e.g jupyterlab-markup.rtfd.io
    • set up preview builds
  • in a PR
    • create .readthedocs.yml
    • capture docs build deps
    • create docs/conf.py
    • create some basic docs examples
• advanced
  • create environment.yml (so the nodejs version can be pinned along with everything else)
  • set up typedoc
  • use jupyterlite
  • add link-checking
  • automate some screenshots with galata/robot

[bollwyvl]

@agoose77 if you fire up the RTD config i can PR the rest!

[agoose77]

Thanks @bollwyvl, done!

[bollwyvl]

Will get a PR up sometime later today.