rustdoc: clean up margin CSS for scraped examples

rustdoc: clean up margin CSS for scraped examples

* This stops applying a margin to the additional example links.
  Because these links are `display: inline`, it doesn't actually do anything.
* This switches from using a margin-bottom with a special exception for
  the examples themselves, plus an additional margin on the hide button,
  to instead using just margin-top on the examples, with an exception for
  the first one.

No user-visible changes should result from this.