Add new Polyfills partial for section index
Add refreshed card-what section for polyfills
Update index What -> Polyfills
Add newly refreshed Custom Elements polyfill doc
Add refreshed HTML Imports doc
Add refreshed Polyfills index
Add newly refreshed doc on Shadow DOM polyfills
Add Polyfills menu item to partials
Temp fix for polyfill URLs.
Add CSS styles for homepage installation stuff.