reference page broken again

Stream: general

[Jz Pan]

Now https://leanprover-community.github.io/mathlib4_docs/references.html only lists 2 references. I believe that the problem occurred at the third item in https://github.com/leanprover-community/mathlib4/blob/master/docs/references.bib whose key uses non-ASCII characters.

[Jz Pan]

Maybe doc-gen4 should report any problem processing references.bib as error.

[Bryan Gin-ge Chen]

We probably also want a CI step in mathlib that catches these issues.