[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