[Redesign] Use Roboto Slab headers on DocumentView Header

Summary: Ref T8099, This feels a little more consistent with the Document.

Test Plan: Review Phriction, Legalpad, Diviner

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13477