WIP: Migrate docs to docusaurus (alpha version)

[ndrsn]

Note: This is a work in progress and should not be merged until the tutorials have been included in the docs, and we've checked all the links still work.

This PR migrates the documentation to use Docusaurus, hopefully making maintenance and changing the documentation easier and more straightforward. It splits up the various sections in canvasDatagrid.md into separate Markdown files (see them in docs/docs/), and adds a docs/jsdoc-to-md.js file that converts the JSDoc annotations to Markdown files (located in docs/reference/ upon generation) using Handlebars templates. A README.md file in docs/ has some more detail.

I think I covered most/all of the sections in canvasDatagrid.md, but please let me know if I missed anything. Also, the generated files in reference/ aren't perfect: I'm not sure they're terribly clear, and if the layout maybe can be improved there. For now, the structure/sections are mostly kept as-is, but I can imagine we'll eventually want to restructure a bit. Any comments or ideas on that are welcome.

What's missing from the docs still is the tutorials: I just haven't gotten around to it yet, and wanted to check in that this the right approach first. The tutorials would need a JS Fiddle or CodeSandbox or similar button as we have now. Ideally, I see each tutorial as a separate markdown file in docs/tutorials/, each with one or more code blocks which automagically are turned into JSFiddles upon page load. Not sure how to get there yet, but that would be the dream, I think.

There's a preview of the documentation here: https://likablestimulatingdatamining.ndrsn.repl.co/

[yaojiu19]

good job