doc: Enable quiet mode for sphinx builds

The sphinx output is very noisy. Add -q to SPHINXOPTS so that only
warnings and errors are shown.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/Makefile

@@ -6,7 +6,7 @@ subdir-y :=
 
 # You can set these variables from the command line.
 SPHINXBUILD   = sphinx-build
-SPHINXOPTS    = -W
+SPHINXOPTS    = -q -W
 SPHINXDIRS    = .
 _SPHINXDIRS   = $(patsubst $(srctree)/doc/%/conf.py,%,$(wildcard $(srctree)/doc/*/conf.py))
 SPHINX_CONF   = conf.py