Additions to The Guide

[Rob Beezer]

Primarily for Alex and Oscar, with no particular rush.

I've been having my assistant attack the empty stubs in The Guide, which is
going pretty well. I think Claude is less verbose than I am. That's a great thing.

Six new sections just went in at:
https://github.com/PreTeXtBook/pretext/pull/2908

I have done a careful review and resultant editing, but there are some items
that Oscar and Alex might wish to edit and submit as PRs. Additional editing is
welcome.

Oscar - Worksheets

* The snippet in the "Basics" part might be worth some review and editing.
Keep the listing to one page in PDF output.
* Did we decide to keep margin specifications for PDF worksheets? I think
these are still useful, even if print-from-HTML is so capable.

Alex - Units of Measure

* Claude wants to say "we won't put a table here, just try stuff and look for
warnings". Are we stable enough to have a table produced? Will it be
maintained? Claude should be able to produce a nice table.

Rob