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
FB bundles wrap warning() calls in __DEV__
Split dev-mode transforms into separate parts:
1) umd+cjs+fb: Wrap warning calls with process.env checks
2) umd+cjs: Replace error messages with minified codes
Also updated transforms to use __DEV__ since it transforms to smaller code after stripEnvVariables is run.
Also renamed 'scripts/error-codes/dev-expression-with-codes.js' -> 'scripts/error-codes/replace-invariant-error-codes.js'
