Adds a new docs website, built with Gatsby JS, to replace the old Jekyll site. Source code for the new site lives in /www (although markdown and YML data still comes from the legacy /docs folder).
Changes to either markdown or website source code can be previewed on Netlify. The react-js bot should automatically add comments to each PR with preview links. (This preview is generated by running the newly-added yarn build:docs command in the root package.json.)
The majority of the changes in this PR are contained within the new /www directory. However some minor modifications have been made to existing content in the /docs directory:
* Modified frontmatter author block to always be an array
* Small markdown formatting tweaks
* Amended implementation-notes.md to include a link to a blog post by Dan Abramov, explaining the difference between components, elements, and instances. An understanding of this distinction is crucial in tracing through Implementation pseudocode, and reading Dan's blog first may ease newcomers into understanding the implementation.
* adjusted wording to maintain stylistic consistency with rest of content, per @aweary's request
* Add more to Codebase Overview
* WIP
* Start a reconciler overview
* Add a few more sections
* todo
* WIP
* Finish it
* Whitespace
* Minor tweaks
* Minor tweaks
