Re #7469: sphinx-build with flag -n (nitpick)

Makefile

@@ -235,6 +235,8 @@ bootstrap-jsons-linux: phony
 
 # TODO: when we have sphinx-build2 ?
 SPHINXCMD:=sphinx-build
+# Flag -n ("nitpick") warns about broken references
+SPHINX_FLAGS:=-n
 SPHINX_HTML_OUTDIR:=dist-newstyle/doc/users-guide
 USERGUIDE_STAMP:=$(SPHINX_HTML_OUTDIR)/index.html
 
@@ -242,7 +244,7 @@ USERGUIDE_STAMP:=$(SPHINX_HTML_OUTDIR)/index.html
 users-guide: .python-sphinx-virtualenv $(USERGUIDE_STAMP)
 $(USERGUIDE_STAMP) : doc/*.rst
 	mkdir -p $(SPHINX_HTML_OUTDIR)
-	(. ./.python-sphinx-virtualenv/bin/activate && pip install -r doc/requirements.txt && $(SPHINXCMD) doc $(SPHINX_HTML_OUTDIR))
+	(. ./.python-sphinx-virtualenv/bin/activate && pip install -r doc/requirements.txt && $(SPHINXCMD) $(SPHINX_FLAGS) doc $(SPHINX_HTML_OUTDIR))
 
 .python-sphinx-virtualenv:
 	python3 -m venv .python-sphinx-virtualenv