doc: sphinx-build prints only warnings or errors

Pass -q to sphinx-build so it only prints what is wrong, not a huge
list of progress information that in most cases is not necessary.

Note that this can be overriden from the environment.

Change-Id: I8735e7ad9a2998734d4e9dda9b3e826ce07b5527
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>