Remove status messages about Sphinx detection

Remove status messages about Sphinx detection

Make FindSphinx.cmake and FindPythonModule.cmake respect the QUIET
option, and pass that to find_package() to not print out information on
every CMake run.  Most people will not care whether these are found or
not, and being silent in all cases is the same approach as is used for
Doxygen.

In master, it could be useful to change at least some of the documentation
build rules such that they require GMX_DEVELOPER_BUILD to be set, and
that could also enable messages about not finding the components needed
for the documentation build, but that is outside the scope of this
change.

Fixes part of #1761 and #1764.

Change-Id: I196f5e66c94fe4247ae28bd230a469acbaad939a