manuals: generate vty reference xml at build time

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

Remove hlr_vty_reference 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-hlr --vty-ref-xml cmdline switch.

Change-Id: I02c9862ced48ce4271b328a0963d5f09086b295c
Depends: I613d692328050a036d05b49a436ab495fc2087ba
Related: OS#5041
diff --git a/Makefile.am b/Makefile.am
index 9a92f2f..e6ba26d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1,9 +1,9 @@
 AUTOMAKE_OPTIONS = foreign dist-bzip2
 
 SUBDIRS = \
-	doc \
 	src \
 	include \
+	doc \
 	sql \
 	contrib \
 	tests \