recursively build doc in all modules before building it for swh-docs

This makes doc building more expensive (because it effectively get
built *twice*), but has the advantage of calling make in all modules, so that
auxiliary pieces of documentation (e.g., DB schemes) are guaranteed to be built
as well.

It also makes sure that everyone who wants to build the global doc has all the
needed dependencies, avoiding inconsistent builds by different people that step
on each other toes.

docs/Makefile

@@ -11,7 +11,7 @@ INSTALL_PERMS = g+rwX
 
 
 html: sphinx/html
-sphinx/html: links-stamp apidoc-stamp images-stamp
+sphinx/html: links-stamp apidoc-stamp images-stamp rec-build-stamp
 
 links-stamp:
 	bin/ln-sphinx-subprojects
@@ -25,6 +25,13 @@ images-stamp:
 	$(MAKE) -C images
 	touch $@
 
+# Build documentation recursively in all packages, to make sure that auxiliary,
+# non-sphinx managed documentation artifacts (e.g., schema diagrams) are also
+# built.
+rec-build-stamp: $(wildcard ../../swh-*/docs/*.rst)
+	$(MAKE) -C ../../ docs
+	touch $@
+
 clean: sphinx/clean
 	bin/ln-sphinx-subprojects --remove
 	$(MAKE) -C images clean