Add relative_entropy to operations documentation

[maxkapur]

This PR adds a line to docs/src/operations.md to cover the relative_entropy(x, y) (KL divergence) function, as requested in this issue: #499

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• Unfortunately, I was unable to build the docs locally to double-check for syntax errors:

ERROR: LoadError: `makedocs` encountered errors (:cross_references, :example_block). Terminating build before rendering.

• I notice that entropy(x) and relative_entropy(x) have opposite signs, despite the former being a special case of the latter. Any interest in a PR fixing this? Which one should flip? This would be a breaking change in either case.

[ericphanson]

I notice that entropy(x) and relative_entropy(x) have opposite signs, despite the former being a special case of the latter. Any interest in a PR fixing this? Which one should flip? This would be a breaking change in either case

The signs are correct, that is how the entropy and relative entropy are defined.