Current stable guides show the "Guides Index" links without underline,
just showing it when it's hovered. This restores the same functionality
for edge guides.

Also remove annoying pointer cursor while accessing guides index,
restoring previously functionality: now the pointer cursor only appears
when hovering a link.

Closes #8107 [ci skip]

This reverts commit 7f536f91.

Reason: If at all this is done, this should be done in rails master.

switches guides code syntax highlighting to the better SyntaxHighlighter, I did my best with the theme, I swear

This reverts commit 4b71fc22.

harmonizes whitespace surrounding special boxes and their inner paragraphs, removes unused selectors as well

If you're a guide writer and want to generate the guides, Run :

  ruby railties/guides/rails_guides.rb

And guides HTML will get generated inside railties/guides/output directory.