feat(09_Using_Tactics): document unfold step and esimp tactic
feat(09_Using_Tactics): document unfold step and esimp tactic

leanprover/tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,avigad/tutorial,leanprover/tutorial,avigad/tutorial,soonhokong/lean-tutorial,cjmazey/fork_leanprover_tutorial,avigad/tutorial,Hodapp87/cs6460_project,fpvandoorn/tutorial,Hodapp87/cs6460_project,soonhokong/lean-tutorial,soonhokong/lean-tutorial,Hodapp87/cs6460_project,fpvandoorn/tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial