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docs referring to outdated mechanics and visuals, and other features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.

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

errors.