R1:1876bef40463

Add CSS and docs for remarkup checkbox list styles

Summary: See D8931. Ref T3945. CSS for the checkmark styles.

Test Plan: {F149713}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T3945

Differential Revision: https://secure.phabricator.com/D8932