This updates the documentation to provide new 1x and 2x screenshots. The

previous screenshots were at 1x and roughly 2.5x, which made them appear

a bit too large compared to other screenshots.
This also updates the alt text for a number of images (though not all).

Width and height information has both been added, now that

beanbag-docutils has been updated to work around some bad width/height

settings from Sphinx.
Some additional screenshots have been added for the comment dialog and

some older diff screenshots.

Built the docs. Viewed each affected page on a MacBook Pro display,

Studio Display, and a 1080p standard-DPI display.
Screenshots in the review request show the updated @2x screenshots within

the context of the pages.