Website: Rename the "nav" Jekyll plugin to "sitenav".

This makes searching for the plugin easier and helps communicate
that the plugin provides builds an index that is both for navigation
and that is a sitemap.
Website: Restore Web Starter Kit icons.

The Web Starter Kit icons are still used by some built-in styles,
such as lists and call-to-action links.
Website: Link to the GitHub repository in the main navigation.
Website: Improve documentation navigation on mobile.
Merge pull request #169 from nicksay/web-icons

Website: Restore Web Starter Kit icons.
Merge pull request #170 from nicksay/web-navigation

Website: Improve documentation navigation on mobile and link back to Github.
Merge pull request #168 from nicksay/web-sitemap

Website: Rename the "nav" Jekyll plugin to "sitenav".