Polishing

This commit includes a few things to make our docs look slicker:

 - Consistent capitalization of DogStatsD in prose (todo: Agent)
 - Fix some broken links
 - Make references to the IRC channel include a link to Freenode's web chat for those without an IRC client
 - Break the styles out into a separate CSS file to improve page load times
 - Make sidebar nav scroll with the viewport
 - Rename the "Getting Started with $thing" page titles to include more information
 - Improve the header and colors (see #77) to match the app and the corporate site

cc @elijahandrews