docs: improve evaluation README with proper links and formatting

[young010101]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

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Link of any specific issues this addresses

[neubig]

Hey @young010101 , really sorry that I didn't see your PR before doing this, but I actually just queued up a big refactor of the eval directory here #5223 . I'll block this PR for now, but we can revisit it when #5223 is merged.

[young010101]

Hey @young010101 , really sorry that I didn't see your PR before doing this, but I actually just queued up a big refactor of the eval directory here #5223 . I'll block this PR for now, but we can revisit it when #5223 is merged.

Thanks for your feedback! I'll wait for #5223 to be merged and then update my PR accordingly.

[mamoodi]

The refactor is finished in case you want to continue with this @young010101

[young010101]

The refactor is finished in case you want to continue with this @young010101

Clear refactor, @mamoodi. I've picked it up already. Could you please review it again?

[mamoodi]

Looks good! Thanks!