feat(04): add sections on relativization and sorts, and set theory
feat(04): add sections on relativization and sorts, and set theory

rlewis1988/logic_and_proof,avigad/logic_and_proof,leanprover/logic_and_proof,avigad/logic_and_proof,soonhokong/logic_and_proof,rlewis1988/logic_and_proof,soonhokong/logic_and_proof,avigad/logic_and_proof,fpvandoorn/logic_and_proof,leanprover/logic_and_proof,fpvandoorn/logic_and_proof,rlewis1988/logic_and_proof,soonhokong/logic_and_proof,fpvandoorn/logic_and_proof,leanprover/logic_and_proof