[RFC] Format style guide.

It bothered me that the style guide was the only thing not in "Neovim style", so I modified it by included the main stylesheet and adapting `guide.css`. The page header now looks like the ones in the clang/translation reports. I removed the green color of `<h2>`, since code blocks use that color as well.

I originally also wanted to move the guide from `/develop/style-guide.xml` to `/develop/style-guide/index.xml`, but adding `redirect_from: /develop/style-guide.xml` to the front matter caused a browser error when accessing `/develop/style-guide.xml`, since the generated redirection code is HTML, not XML.
For Doxygen, clicking the logo would point to `/doc/dev/index.html`, so the idea was to get to the index page of the current report from the logo. Since all reports & the style guide are one-page only, this doesn't make much sense.. maybe remove the link completely? The reports are somehow sub-websites of neovim.org (different navigation bar), so I'm not sure about changing this to `/`.