Move code examples in remarkup to grey backgrounds

Summary: This should make code easier to read. Right now large blocks get a really bright treatment, and this should tone it down. Also upped the spacing a bit.

Test Plan:
Review Remarkup Examples from Diviner, also some test code blocks and counter examples.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15141