Store a backup of the HTML documentation in docs/htm
English doc RSTification part 1

The main directory and advancedtopics subdirectory

The image resources for the html files have also been moved to not
be on a higher tree level where images for all subdirs of the
English docs are stored. Now the ones for a particular subdir
reside in the pictures/ directory inside of that subdir.

The reason for this is that restview has issues with absolute paths,
and while Sphinx might be able to handle it, I don't know if
rst2html does. So err on the side of caution and put the relevant
images beneath the directory where they get called from.

The page generated from faq.htm is now located at faq_historical.rst

Reason being, it appears that most of the faq_ pages and possibly
a couple other pages in maindir/ were the result of faq.htm being
broken apart into separate pieces, because there's a ton of
duplicate content in faq.htm.

I'm not really enthused to do all that work all over again, when
the information is mostly contained in other files. So it'll be
left as-is, waiting to be deleted upstream at a later time. If
there's something to salvage from it, then that can be decided
at that point.
English doc RSTification part 2: corefilters: A-C
English doc RSTification part 2: corefilters: D-F