docs: make sure images are built/cleaned on sphinx targets

Related D454

docs/Makefile

 include ../../swh-docs/Makefile.sphinx
+-include Makefile.local

docs/Makefile.local

+sphinx/html: images
+sphinx/clean: clean-images
+
+images:
+	make -C images/
+clean-images:
+	make -C images/ clean
+
+.PHONY: images clean-images
+
+
+# Local Variables:
+# mode: makefile
+# End:

[Phabricator Migration user]

Some references in the commit message have been migrated:

• D454 is now !14 (closed)