Convert FilterSDK docs to ReStructuredText

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 temporarily.
A later commit will move them into the same directory as all
the other HTML backups.

This is a straight conversion of the original docs into RST, no
updating of the content has been performed, with few - if any -
exceptions.
Store a backup of the HTML documentation in docs/htm
Store FilterSDK HTML in docs/htm like everything else.
Convert the English documentation to ReStructuredText.

The image resources for the html files have 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 content has not been updated, with few - if any - exceptions.
So if a link doesn't work, it probably doesn't work in the
original docs either. Fixing that kind of thing is a task for
general doc updating, not RST conversion.
Add rst_migration_notes.txt that outlines RST formatting style.