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 `<iframe>`.

After that I used a dynamically resizing `<iframe>` to embed the docs within our Wordpress theme.  This will make it very easy to automatically update the docs from the build server in the future.

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.

This is all part of the big documentation project https://github.com/mono/MonoGame/issues/2378.