Put an up-to-date Makefile in docs/Makefile

It doesn't do anything useful yet, but it works with the new build system

docs/Makefile

@@ -1,26 +1,4 @@
+dir = docs
 TOP = ..
-include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/sub-makefile.mk
 
-SUBDIRS = man docbook-cheat-sheet users_guide ext-core storage-mgt
-
-PAGE = index.html
-
-install-docs :: $(PAGE)
-	$(INSTALL_DIR)                          $(DESTDIR)$(htmldir)
-	$(INSTALL_DATA) $(INSTALL_OPTS) $(PAGE) $(DESTDIR)$(htmldir)
-
-.PHONY: binary-dist binary-dist.doc.%
-
-binary-dist: $(foreach SUBDIR,$(SUBDIRS),binary-dist.doc.$(SUBDIR))
-ifeq "$(WHERE_AM_I)" ""
-	echo "I don't know where I am" >&2
-	exit 1
-endif
-	echo $(WHERE_AM_I)/Makefile >> $(BIN_DIST_LIST)
-	echo $(WHERE_AM_I)/$(PAGE)  >> $(BIN_DIST_LIST)
-
-$(foreach SUBDIR,$(SUBDIRS),binary-dist.doc.$(SUBDIR)): \
-binary-dist.doc.%:
-	$(MAKE) -C $* binary-dist WHERE_AM_I=$(WHERE_AM_I)/$*
-
-include $(TOP)/mk/target.mk