[CHANGE] side-by-side in the schema

[Rob Beezer]

* A long time ago, we allowed a one-panel #sidebyside as a way to get some
layout control cheaply. That is no longer necessary - things like an #image can
now have margin and width specifications.

So a one-panel #sidebyside has been deprecated. Two panels or more are
required. No changes yet to existing code, you will simply get no-rush warnings.

* Also, there was a bit of recursion hidden in the schema. Because #figure can
be both a child, AND a parent, of a #sidebyside you could author:

figure
sidebyside
image
figure
sidebyside
image
image

As you could perhaps guess, that does not work out very well in the end. And
now it is gone from the schema. Not really a deprecation - we are calling it a bug.

* The language of a #sidebyside is used as layout control for #slate in an
#interactive, there have been some tweaks to the schema so that works out better.

https://github.com/PreTeXtBook/pretext/pull/3182

Rob