Left and right margins for "sidebyside"

[Rob Beezer]

In an un-related post, Joe C was using a side-by-side panel with an empty "p" to
shove the second panel to the right.

Now you can also specify @margins with a space-separated list of two
(different) percentages, a left margin and a right margin.

https://github.com/rbeezer/mathbook/issues/698

Fully-documented in the Author's Guide (with the rest of "sidebyside"). I think
I'm done with "sidebyside" for a while.

Rob