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
3 files changed