This is the documentation update pull for the 4.9 merge window.

The Sphinx transition is still creating a fair amount of work.  Here we
have a number of fixes and, importantly, a proper PDF output solution,
thanks to Jani Nikula, Mauro Carvalho Chehab and Markus Heiser.

I've started a couple of new books: a driver API book (based on the old
device-drivers.tmpl) and a development tools book.  Both are meant to show
how we can integrate together our existing documentation into a more
coherent and accessible whole.  It involves moving some stuff around and
formatting changes, but, I think, the results are worth it.  The good news
is that most of our existing Documentation/*.txt files are *almost* in RST
format already; the amount of messing around required is minimal.

And, of course, there's the usual set of updates, typo fixes, and more.
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