Finer points of laying out larger LaTeX boxes

[Rob Beezer]

This is for those interested in spacing out blocks (and similar) in LaTeX
output. Which means Alex and Oscar, and maybe some others who have tangled with
the when and where of using \par, and especially \leavevmode.

Oscar just brought some very good improvements to #sidebyside with some surgical
\par about.

https://github.com/PreTeXtBook/pretext/pull/3186

I asked Claude if, generally, we could find a less fiddly approach to all of
these adjustments sprinkled through the code. There is a report in:

https://github.com/PreTeXtBook/pretext/issues/3192

And I made a few adjustments - the most obvious one is a surfeit of \par between
panels of a #sidebyside. These replicate all the ones Oscar added, and include
a bunch more, many of which simply don't cause any harm. But doing that greatly
simplified a chunk of code, and maybe avoids some bugs we had not noticed yet.

https://github.com/PreTeXtBook/pretext/pull/3190

Rob