make-doc: use the docs-assets target instead of the full-sphinx-build docs

so we do not build each swh-* twice with unavaoidable sphinx wrnings when
building each repo locally.

Makefile.sphinx

@@ -42,6 +42,8 @@ apidoc-stamp:
 	find $(shell pwd)/apidoc -name "*.rst" | xargs sed -i '1i:orphan:\n'
 	touch $@
 
+assets:
+
 .PHONY: clean clean-apidoc
 clean: sphinx/clean clean-apidoc
 clean-apidoc:

docs/Makefile

@@ -33,7 +33,7 @@ images-stamp:
 # non-sphinx managed documentation artifacts (e.g., schema diagrams) are also
 # built.
 rec-build-stamp: $(wildcard ../../swh-*/docs/*.rst)
-	$(MAKE) -C ../../ docs
+	$(MAKE) -C ../../ docs-assets
 	touch $@
 
 clean: sphinx/clean