So that the gtkdoc-rebase step fixes the URLs to contain the correct
links; e.g.:
$ make dist V=s
make[8]: Entering directory '/home/aleksander/Development/foss/ModemManager/docs/reference/libmm-glib'
make \
top_distdir="../../../ModemManager-1.15.0" distdir="../../../ModemManager-1.15.0/docs/reference/libmm-glib" \
dist-hook
../ModemManager/ -> https://www.freedesktop.org/software/ModemManager/doc/latest/ModemManager/ (914)
../gio/ -> https://developer.gnome.org/gio/unstable/ (2564)
../glib/ -> https://developer.gnome.org/glib/unstable/ (5534)
../gobject/ -> https://developer.gnome.org/gobject/unstable/ (499)
make[8]: Leaving directory '/home/aleksander/Development/foss/ModemManager/docs/reference/libmm-glib'
Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/298