Modernize and flesh out the diff viewer documentation.

Review Request #962 — Created Nov. 14, 2017 and updated — Latest diff uploaded

Review Board
The diff viewer documentation has gotten a bit stale, with parts of the
docs referring to outdated mechanics and visuals, and other features
just not represented. On top of that, the organizational structure of
the docs didn't hold up so well over time, with information being in
scattered places.

This redoes the documentation to provide a better structure for the
information, to cover missing features, and to update existing text to
reflect how the diff viewer now works.

The docs are now broken into four top-level sections: Navigating Diffs,
Reading Diffs, Seeing More of a File, and Working with Comments.

Navigating Diffs covers the file listing, revision selection, and
keyboard shortcuts.

Reading Diffs covers the side-by-side diff view, moved line indicators,
indentation highlighting, and replaced line highlighting.

Seeing More of a File covers diff expansion, viewing deleted content,
and downloading the full content of files.

Working with Comments covers the basics of comments in the diff viewer,
creating new comments, and reading existing published comments.

New screenshots have been added all throughout, helping to provide a
better visual tour. The old "parts" image breaking down the structure of
the diff viewer has been removed, though, in favor of just more visual
aids throughout. This image was harder to keep updated, harder to have
in both standard and high-DPI format, and didn't ultimately add much
more information when considering the rest of the screenshots and other
refinements we've made to docs since it was introduced.

Along with this, the comment dialog images have been replaced. The
recent revisions of these had some pixel alignment and color palette
issues, which these fix.

Testing Done:
Built the docs. Checked for build errors, broken links, and spelling

Reviewed at