added lesson 6, fig. 8-queens, version 2020-21, recursivity codes

lessons/contenu_cours_6.md

lessons/figures/fig_recursivite_8_reines.png

source_codes/recursivity/lissage.c

source_codes/recursivity/rec_binary.c

source_codes/recursivity/rec_factorielle.c

source_codes/recursivity/rec_fibonacci.c

source_codes/recursivity/rec_pgcd.c

source_codes/recursivity/rec_ppcm.c

source_codes/recursivity/rec_puissance_indienne.c

source_codes/recursivity/rec_reines.c

source_codes/recursivity/rec_reines_skel.c