Created doc/doc/extra_files directory 

...for files that should show up in root of `clawpack.github.io`, and modified `conf.py` to add this directory in `html_extra_path`

The plan is to have www.clawpack.org point to http://clawpack.github.io and copy `clawpack/doc/doc/_build/html` to the top level of `clawpack/clawpack.github.com` rather than to `clawpack/clawpack.github.com/doc`.

This way all webpages will be "built" by Sphinx and we can eliminate the `doc` subdirectory of `clawpack/clawpack.github.com`.

Copies some files and re-direct links to this directory.