Upgrade to typedoc v0.20

[donmccurdy]

Fixes #182.

Several good things here: Typedoc will now let us document the extensions and lib packages, not just core, on the same site. And the CLI seems to be a bit faster and nicer. But, there are some breaking things to work through here:

• Various things missing from the theme. Review Usage with typedoc@0.20.0-beta.24 mipatterson/typedoc-plugin-pages#47 and fix: Fixed "The partial literal could not be found" mipatterson/typedoc-plugin-pages#41.
• The markdown pages I've tried to include (Concepts, Library, Extensions, ...) are getting dropped. Might need to lean on https://github.com/mipatterson/typedoc-plugin-pages here.

Maybe also have a look at the @category tag usage, it might get rid of the need to manually write the TOC here. TypeStrong/typedoc#564.

[donmccurdy]

A few more thoughts:

• Should probably start by just upgrading, and add the other packages to the docs in a separate PR.
• When that happens, links to many existing pages will change, so we may want Vercel to handle the redirects.

[donmccurdy]

Waiting for TypeStrong/typedoc#1580.