FilterSDK doc RSTification

ReStructuredText is almost completely plain-text, and easier to
edit than HTML.  It also has the benefit of being parsed directly
by Github, so that formatted documentation can be viewed while
browsing the Git repo itself.

There will be one commit for every directory of RSTified docs.

Moves the HTML docs into the htm/ subdirectory while the RST docs
are proof-checked.  html2rst.py only takes you about 80% of the
way; there's syntax errors and other miscellaneous formatting nits
that need to be adjusted. This is a straight conversion of the
original docs into RST, no updating of the content has been
performed, with few - if any - exceptions.

The resulting RST docs can be converted to HTML using rst2html, or
[in theory] to PDF with rst2pdf, or [again, in theory] to manpages
with rst2man.  Practically speaking, getting PDFs and manpages out
of it will take some syntax tweaking (really, there will probably
need to be a revised set to generate manpages from, and the PDFs
should probably be generated from those). But rst2html will result
in perhaps 99-100% identical HTML docs to the originals.
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
English doc RSTification part 3: externalfilters: pre-treat

fft3dfilter.htm causes an error with html2rst.py that never
happened with any of the other conversions. Investigate.
English doc RSTification part 3: externalfilters: A-C