Add doc page for dfdlx extension features

Daffodil has numerous dfdlx features.

These do have documentation scattered on the developers wiki under design notes, and intermixed with other things that aren't documentation users would need. Users aren't going to be able to find this.

Since these are dfdlx features, a wiki page with links to their doc is the right thing, but we should have a link on the user web site that reaches this summary page, with links to all the dfdlx extension page wiki pages.  Copious warnings about "subject to change without backward compatibility support." can then be clearly displayed, and the dfdlx features can be grouped into those we want people to experiement with, and those we don't (which we might possibly just decide not to mention at all.)

The above is intended to add minimum work to adding a dfdlx extension to DFDL while giving users a clue about dfdlx features they may see in a DFDL schema, or need to use.