Conversion of English documentation to RST

After a little more than a month, it's finally done.  Style should be reasonably consistent internally.

I probably (almost certainly) won't be doing any of the other languages, simply because I found that there were technical issues to overcome that native speakers of those languages would be better able to handle.  My worknotes on style and conversion methodology are stored in distrib/rst_migration_notes.txt, and explains these technical issues I mentioned a bit more thoroughly.

The content of the FilterSDK and English docs have not been updated, since that's not a task for RST conversion - that should be done in general doc updates (this includes links that may point to dead addresses - the original docs wouldn't work for those either).  Any minor changes of the content were almost always so that it would be okay with RST syntax, or may have been otherwise cosmetic to help improve readability (the Changelog/Version history lists were all changed to grid tables).  There may have been one or two really weird and irrelevant lines in a couple files that I deleted during the process - I think they were saying something about 'plugin autoloading doesn't exist at all'.

In the meanwhile, backups of all the HTML docs are stored in docs/htm, so that the other languages can be converted to RST at a later time without compromising the existing HTML docs.

The one single issue that remains is that I haven't looked at which files need which CSS stylesheets, which is important for using rst2html to convert them back to HTML docs.  But seeing as how that is generally more relevant for Linux (and would need updates to CMakeLists and whatnot to detect Python and docutils), and that the original English HTML docs are still available in the source tree in docs/htm, this isn't so big of a deal.  The vast, vast majority of the work is already done.