[Redesign] Clean up remarkup header sizes
Summary: Ref T8099, these grew during the Oswald days, this calms them down a bit.
Test Plan: Review fonts sizes in both descriptions and in documents.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T8099
Differential Revision: https://secure.phabricator.com/D13364