Documentation Style Fixes

This PR improves the SharpDoc "MonoGame" style to make the documentation feel like it is part of the MonoGame.net website.

 - Removed header.
 - Removed footer.
 - Removed scrollbars from inner iframes.

After that I used a dynamically resizing `<iframe>` to embed the docs within our Wordpress theme.

It looks pretty great IMO...

  http://www.monogame.net/documentation/

I still want to improve the style more moving forward:

  - Scroll to top of page when switching content.
  - Full URLs to content for easy sharing.
  - Remove C++ syntax tabs.
  - Figure out what to do for mobile.