Add Doxyfile.trx.in (based on Doxyfile.core.in from libosmocore.git, upgraded using 'doxygen -u' to get rid of obsolete tags) and the usual rules to build and install the HTML documentation, as well as the --disable-doxygen configure option, like in other libosmo-* projects. Also document the missing buf/len parameters of osmo_trxc_clock_ind_parse(), fixing the only Doxygen warning.
