docs dir reorg

I just reorganized the docs directory, now called "docs". I thought
it would be easier to get a totally clean start, and the project is
early enough that I don't think losing version history is too
important. Basically, once we learned we needed to build everything
at once to get the links working, all of the stuff Darren initially
set up and then changed on my request was basically right so I
reverted to a similar organization. Actually, I checked out the
python docs and tried to follow their model as closely as possible,
including the use of the rst extension. Since ipython, numpy and
scipy are moving or have moved to a sphinx documentation system, the
more we all follow the same conventions the easier it will be to mix
and match documents, build books out of them, etc...

Every subdoc project has a index.rst file and several other rst files
(since python uses rst I figures we should too). The subdoc dirs are
"users", "devel", and "api". I created some symlinks in the top level
"docs" directory in case we need to refer to mpl-data or the examples
subdir in the rst documents w/o having to have crazy relative paths
that may change. This is explained in the documenting_mpl.rst file.

Let me know if you have any problems...