gnu: Move Sphinx and friends to (gnu packages sphinx).

gnu: Move Sphinx and friends to (gnu packages sphinx).

* gnu/packages/python-xyz.scm (python-sphinxcontrib-websupport,
python2-sphinxcontrib-websupport, python-sphinx, python2-sphinx,
python-sphinx-gallery, python2-sphinx-gallery, python-sphinx-rtd-theme,
python2-sphinx-rtd-theme, python-guzzle-sphinx-theme,
python2-guzzle-sphinx-theme, python-sphinxcontrib-newsfeed,
python-sphinxcontrib-programoutput, python2-sphinxcontrib-programoutput,
python-sphinx-repoze-interface, python2-sphinx-repoze-interface,
python-sphinx-cloud-sptheme, python2-sphinx-cloud-sptheme,
python-sphinx-alabaster-theme, python2-sphinx-alabaster-theme,
python-sphinx-me, python-sphinxcontrib-svg2pdfconverter): Move to ...
(gnu):
* gnu/packages/sphinx.scm: ... here.  New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Adjust accordingly.
* gnu/packages/admin.scm, gnu/packages/bioinformatics.scm,
gnu/packages/calendar.scm, gnu/packages/cluster.scm,
gnu/packages/databases.scm, gnu/packages/dav.scm, gnu/packages/disk.scm,
gnu/packages/django.scm, gnu/packages/elf.scm, gnu/packages/emacs-xyz.scm,
gnu/packages/emulators.scm, gnu/packages/finance.scm, gnu/packages/image.scm,
gnu/packages/libffi.scm, gnu/packages/mail.scm, gnu/packages/mpd.scm,
gnu/packages/openstack.scm, gnu/packages/pdf.scm, gnu/packages/python-web.scm,
gnu/packages/search.scm, gnu/packages/statistics.scm,
gnu/packages/storage.scm, gnu/packages/sync.scm, gnu/packages/terminals.scm,
gnu/packages/tls.scm, gnu/packages/web.scm: Adjust module imports.