Update the Sphinx installation instructions in the documentation
contributors guide to reflect the fact that in the 1.4+ versions
of Sphinx the ReadTheDocs theme must also be installed. Previously,
in version 1.3.x, it was installed by default.
Also change 'yum' to 'dnf' for package installations.
Signed-off-by: John McNamara <john.mcnamara@intel.com>
4b677a3b60