[dune-functions] Documentation

[Oliver Sander]

Hi guys,

if we want to convince people of dune-functions next week we need at least a
minimal amount of documentation.  As a start, I have written a short web page
presenting the module, and uploaded it to dune-web.  Please read it and append/fix
it if you think that's necessary.

We may also have a look at the doxygen documentation of dune-functions.
Certainly there's room for improvement here :-)  Can somebody please link the
new web page to the doxygen stuff?  I don't know how.

Finally, I have also started to write prose explaining how the stokes-taylorhood.cc example
works. I wrote that for the book, and it actually describes a separate implementation
which uses MultiTypeBlockMatrices for the linear algebra.  If I find the time I may
factor out that text into a separate document and upload it to dune-functions itself.
Best,
Oliver