manuals: generate vty reference xml at build time

Move 'doc' subdir further down to "make sure" the osmo-mgw binary is built
before the docs.

Remove mgw_vty_reference.xml from the source tree.

In manuals/Makefile.am use the new BUILT_REFERENCE_XML feature recently added
to osmo-gsm-manuals, and add a build target to generate the XML using the new
osmo-mgw --vty-ref-xml cmdline switch.

Depends: I613d692328050a036d05b49a436ab495fc2087ba (osmo-gsm-manuals)
Change-Id: I526af21134087e2b43b9ada59c93f636ae242e24
diff --git a/Makefile.am b/Makefile.am
index 1255210..ae063da 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -9,10 +9,10 @@
 	$(NULL)
 
 SUBDIRS = \
-	doc \
 	include \
 	src \
 	tests \
+	doc \
 	contrib \
 	$(NULL)