[CHANGE] Division companions for traditional divisions

[Rob Beezer]

I have always envisioned the #introduction and #conclusion to a division (eg
#chapter, #section, etc) as just a #p or two, explaining why the subsequent
subdivisions were grouped together in the division (or what they accomplished).
A #objectives and a #outcomes are similar, more formal, instances of something
of the same purpose. Code is calling these "division companions".

* These no longer show up as text on an HTML "summary page" (the page of links,
that formerly just showed links to subdivisions, and the actual content of a
division companion). In particular, there will be links labeled "Introduction"
and "Conclusion" (localized). And #objectives and #outcomes will have text that
looks like their usual headings.

* #introduction and #conclusion allowed an optional #title. That was a mistake.
As a first step toward de-emphasing #introduction and #conclusion, they will
not carry titles, and any that you have authored now will be be ignored. You
will get warnings.

* You may want to put/move preliminary content into an initial subdivision with
an appropriate title. You may want to re-title an initial subdivision that
right now also has the (hard-coded) title "Introduction". You may want to get
rid of an #introduction altogether. There are a variety of ways to adjust - the
principle is that an #introduction should not have much mainline content, just
an explication of its parent division's purpose.

Questions and comments on -support or -dev.