Move from doc/manuals to manuals/OsmoNITB as we have two more manuals
(OsmoMGCP and OsmoNAT) that need to be moved to manuals/. Make the
manuals build without a toplevel configure.ac file from autotools:
$ cd manuals/OsmoNITB
$ make
openbsc/configure.ac is not used to integrate the manuals, as it would
be extra effort to integrate all three of the manuals there. This is
not worth it for the legacy codebase.
Related: OS#3385
Change-Id: I0036ad8dbecd23695305ac491c7ee87caa44c7f9
