Add new docs website (#10896)

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