[DEPRECATE] "hacked in" Runestone HTML

[Rob Beezer]

When we started converting Runestone ReST books to PreTeXt, support evolved for
the different exercise types. In order to expedite the process we had a
(temporary) device where you could place raw Runestone-generated HTML into your
PreTeXt HTML. (And only HTML, no hope of any rendering for static.)

You will know if you are doing this because you have exercises with a
@runestone attribute that points into a "rs-substitutes.xml" file.

Consensus is that this device is no longer necessary since there is now
supported PreTeXt syntax for the various exercise types. So:

* exercise/@runestone will generate a typical deprecation message.

* Examples have been removed from the sample book.

* Code supporting this has been removed. An #exercise will remain, so
numbering, etc won't be thrown off. But the contents, dynamic OR static, just
says there is nothing to see here, and suggests the reader let the author know.

* Authors will want to re-write any remaining problems using this device.

Questions to -support, -dev, or perhaps the Runestone support channels.

Rob