Euclid preparation: X. The Euclid photometric-redshift challenge by Desprez, G, Paltani, S, Coupon, J, Almosallam, I, Alvarez-Ayllon, A, Amaro, V, Brescia, M, Brodwin, M, Cavuoti, S, De Vicente-Albendea, J, Fotopoulou, S, Hatfield, PW, Hatfield, PW, Ilbert, O, Jarvis, MJ, Longo, G, Rau, MM, Saha, R, Speagle, JS, Tramacere, A, Castellano, M, Dubath, F, Galametz, A, Kuemmel, M, Laigle, C, Merlin, E, Mohr, JJ, Pilo, S, Salvato, M, Andreon, S, Auricchio, N, Baccigalupi, C, Balaguera-Antolinez, A, Baldi, M, Bardelli, S, Bender, R, Biviano, A, Bodendorf, C, Bonino, D, Bozzo, E, Branchini, E, Brinchmann, J, Burigana, C, Cabanac, R, Camera, S, Capobianco, V, Cappi, A, Carbone, C, Carretero, J, Carvalho, CS