Feature/design changes

These changes make a number of improvements, mostly typographical, to improve readability and to render properly on small devices.

* Universal increase in font size
* Use same font as ss.org for consistency
* Remove unnecessary visual noise (borders, box shadows, etc)
* Use Google PrettyPrint syntax highlighter for improved readability of code sections
* Fully responsive, with hamburger menu
* Add "section" linking mode to menu items
* Add masthead for breadcrumbs, introduction
* Improved styles for notice/info/warning messaging

All changes have been peer-reviewed by Paul.