Fixes multiple errors while making docs:

    Could not import extension sphinxcontrib.rubydomain (exception: No module named 'sphinxcontrib')

and

    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttp.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttpd.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttpx.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'h2load.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'programmers-guide'