Vendor the css and refer to it instead

We were linking to `http://jashkenas.github.io/docco/resources/linear/docco.css` but that has changed a lot and the examples all look horrible now.  This should fix it.
This hardcoding seems unnecessary, since we control what gets written out and have the file at `ex/css.css`, simply `href="css.css"` should be enough, no?

I hear rumours that people other than libgit2 use this project, so they'd probably appreciate it if we didn't hardcode our url into it.