c-t-e doxygen dead link?

Hi Daniel, it looks like the doxygen link on
<http://clang.llvm.org/extra/&gt; is dead. Could you look into it? The
`doxygen` makefile target needs to be run, and it places the doxygen
output alongside the sphinx output in a doxygen directory (i.e.
index.rst -> `_build/html/index.html` and doxygen puts its main page
in `_build/html/doxygen/index.html`).

Thanks,

-- Sean Silva