Fix links to informal quint

[rach-id]

The CI reports the following dead links:

  [✖] https://github.com/informalsystems/quint/blob/main/tutorials/README.md → Status: 404
  [✖] https://github.com/informalsystems/quint/blob/main/doc/quint-cheatsheet.pdf → Status: 404
  [✖] https://github.com/informalsystems/quint/blob/main/doc/builtin.md → Status: 404