FilterSDK doc RSTification part 1: A-C

Once all of them are done this will get squashed into a single
commit, one per topical directory.

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.

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.
FilterSDK doc RSTification part 2: D-P