Fix "source" links in docs

[khieta]

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Bug Category

Other

Describe the bug

As pointed out #294, the "source" links are broken in the auto-generated docs. E.g.,

https://cedar-policy.github.io/cedar-spec/docs/Cedar/Thm/Slicing.html

links to

https://github.com/cedar-policy/cedar-spec/blob/0cd7497ba2674d087179bcac5432923547c88e5a/Cedar/Thm/Slicing.lean#L47-L58

(which isn't a valid path, since the Lean code should be nested inside cedar-spec/cedar-lean).