To improve wrapping behavior of rendered README files, don't use "PHUIDocumentView" in Diffusion

Summary:
See PHI1268. We currently do some weird width handling when rendering Diffusion readmes in a document directory view.

I think this came from D12330, which used PHUIDocumentViewPro to change the font, but we later reverted the font and were left with the DocumentView. Other changes after that modified DocumentView to have fixed-width behavior, but it doesn't make much sense here since the content panel is clearly rendered full-width.

Today, the DocumentView is a more structural element with methods like setCurtain(). Just get rid of it to simplify things, at least as a first step.

Test Plan:
Before:

After:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20536