feat: Migrate API user docs to official Usage docs

[kamranayub]

Closes #25
Fixes #3
Closes #6 (/docs)
Fixes #30

Migrate our old crufty docs to the top-level of excaliburjs.com/docs and polish it up! 🎉

Preview the Docs

https://excaliburjs-preview.netlify.app/docs/intro

Changes

• Migrate user docs to main docs site
  • Clean up links
  • Clean up organization
  • Update example snippets
  • Update outdated info
• Add rehype-typedoc module to transform Markdown symbol links like in Typedoc, e.g. [[Engine.start]]
• Add gatsby-source-typedoc to ingest edge typedoc JSON at build time
• Update docs page template

TODO

• Figure out how to resolve symbol references to TypeDoc generated docs (gatsby plugin?)
• Update outdated examples / info
• Polish links and symbols
• Embed examples from https://excaliburjs.com/examples where applicable

[kamranayub]

TypeDoc does allow you to build using --json output. We could maybe use this by sucking it up via a Gatsby plugin, then adding a custom element/component to the Markdown plugin for using it as a GQL source to find and search for symbols.

So like [[Engine]] will resolve to the URL classes/_engine_.engine.html and any other symbols can be resolved like that (with build errors if they are missing).

[kamranayub]

Currently it properly replaces Typedoc links with placeholder <a> tags:

Just need to enhance the rehype-typedoc transformer to take in typedoc and search/find the symbols.

The way Typedoc does it is by searching the scope of the current module and then moving upwards. Since the docs are disconnected from typedoc, it will probably have to rely on more fully-qualified symbol names, which we could update during the migration. Using CSS I could highlight any links with bad/missing symbol references.

[kamranayub]

Writing tests, to use jest 26 we need #42