doc/Makefile: run sphinx in serial mode

Unfortunately, using pydata_sphinx_theme extension generates warnings in
sphix processing. These warnings cause documentation build to be
considered a failure:

WARNING: the pydata_sphinx_theme extension is not safe for parallel writing
WARNING: doing serial write
....
build finished with problems, 2 warnings.
make: *** [Makefile:24: html] Error 1

Since the build is already doing a serial write, enforce it from start.

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Francisco Trivino <ftrivino@redhat.com>

doc/Makefile

@@ -5,7 +5,7 @@
 # from the environment for the first two.
 PYTHON        ?= python3
 VENVDIR       = ./.venv
-SPHINXOPTS    ?= -W --keep-going -j auto
+SPHINXOPTS    ?= -W --keep-going -j 1
 SPHINXBUILD   ?= PATH=$(VENVDIR)/bin:$$PATH sphinx-build
 SOURCEDIR     = .
 BUILDDIR      = _build