feat(basics): add "proof by case analysis" section
feat(basics): add "proof by case analysis" section

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