feat(order/monotone): Functions from/to subsingletons are monotone (#10903)
feat(order/monotone): Functions from/to subsingletons are monotone (#10903)

A few really trivial results about monotonicity/antitonicity of `f : α → β` where `subsingleton α` or `subsingleton β`.

Also fixes the markdown heading levels in this file





leanprover-community/mathlib,leanprover-community/mathlib