feat(03_Propositions_and_Proofs.org): expand explanation of propositions-as-types
feat(03_Propositions_and_Proofs.org): expand explanation of propositions-as-types

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