Clean up Remarkup Code Block Headers
Summary: Removes the special background and fonts. Uses just a simple bold header.
Test Plan:
Checked out installation guide and remarkup guide in Diviner. Looks cleaner.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6904
Differential Revision: https://secure.phabricator.com/D11869