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
Rendering to `document.body` in the examples is conveniently short, but it can
be misleading at the same time, especially for newcomers.
While it's possible to render React components to `document.body`, any 3rd
party scripts can also mess up with it, and it can have unintended consequences
and be source of difficult-to-trace bugs.