doc/overview: mention the presence of archive.d.o releases
doc/overview: mention the presence of archive.d.o releases

vivekanand1101/debsources,oorestisime/debsources,matthieucan/debsources,clemux/debsources,zacchiro/debsources,oorestisime/debsources,sophiejjj/debsources,clemux/debsources,devoxel/debsources,Debian/debsources,vivekanand1101/debsources,nonas/debian-qa,zacchiro/debsources,devoxel/debsources,sophiejjj/debsources,Debian/debsources,Debian/debsources,clemux/debsources,vivekanand1101/debsources,matthieucan/debsources,clemux/debsources,matthieucan/debsources,sophiejjj/debsources,zacchiro/debsources,nonas/debian-qa,oorestisime/debsources,oorestisime/debsources,vivekanand1101/debsources,devoxel/debsources,vivekanand1101/debsources,Debian/debsources,clemux/debsources,matthieucan/debsources,Debian/debsources,sophiejjj/debsources,devoxel/debsources,oorestisime/debsources,matthieucan/debsources,sophiejjj/debsources,zacchiro/debsources,nonas/debian-qa,nonas/debian-qa,zacchiro/debsources,devoxel/debsources