Document diff mode

[caraitto]

The diff mode feature I see on pull requests is very useful. Here's how I access it:

1. Modify a spec, build the diff. For instance, click on the "Diff" link in the "Preview | Diff" section of Spec: Fix error and not requested cases around trusted bidding signals WICG/turtledove#1180. (I'm not sure how to do this from the command line, it doesn't seem to be documented -- maybe it's another tool and not Bikeshed that does this, or maybe not? I couldn't figure it out looking at the GitHub build log for that PR.)

The Bikeshed documentation doesn't mention this at all though, AFAICT? Neither does bikeshed --help.

It would be nice to mention how to build a diff from the command line. Also, it would be nice to document keyboard shortcuts, like "j" and "k", and the next, back and 1 of N UI widget at the bottom right.

If this isn't a Bikeshed feature, do you know how the diff page gets built by the GitHub action?