Docusaurus-powered documentation website

[cspotcode]

https://typestrong.org/ts-node

Implements #1099

See also: TypeStrong/discussions#6

What if we merge this now so that the website gets published automatically, then iteratively improve the docs over time?

In other words, the website will be live but incomplete, and we won't link to it until it's ready.

[codecov]

Codecov Report

Merging #1207 (69f041e) into master (237ea5a) will decrease coverage by 0.25%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
src/index.ts	78.81% <ø> (-0.50%)	⬇️

[cspotcode]

The sidebar navigation, table of contents, and "prettiness" of the site may encourage people to read the documentation, more easily find the answers they need. Plus I think it frees us up to create dedicated pages with tsconfig samples, examples, stuff like that.

But for npm we still want to have a README. So maybe there's some easy, hacky way to build up the README from the various docs pages, essentially copy-pasting them together via a build script. Then we can write docs for the docsite and still get a meaningful README published to npm.

[cspotcode]

Added typedoc API docs since it was so easy to do. May prove helpful to link people to an HTML rendition of our programmatic options.

Added a "under construction" message to the first page.

Marked as "ready for review" because I think this can safely be merged today, then we can improve the pages over time until we deem the site ready to recommend to new users.

[cspotcode]

Merged to master, and I'll track future website & docs work in follow-up issues and PRs.