Automated Dead Link Detection

[compiler-errors]

Would be very cool if we detected dead rustdoc links and stuff and filed issues for them, or at least failed CI for them. This would make sure we can both take advantage of heavy cross-linking w/o risking pages silently rotting b/c of refactors.

[tshepang]

We stopped making bad links fail CI since it caused too much pain for contributors (because the breakages are that frequent). We still run the check daily though, but we hardly go have a look to see what it broke.