mathlib3

849a5f9b - feat(docs,ci): move overview, undergrad, and 100 theorems lists from website (#4016)

feat(docs,ci): move overview, undergrad, and 100 theorems lists from website (#4016) See conversation at https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/website.20overview/near/208659351 We'll store these lists in mathlib so that we can catch breakage as soon as it happens, rather than continually repairing the website build. This PR adds the lists and a CI step that checks that every declaration name appearing in the lists actually exists in the library. Co-authored-by: Rob Lewis <rob.y.lewis@gmail.com>