Add gallery example for Mutual information

[seanbudd]

Mutual information would be worth a gallery example for itself I think. This PR is already enormous, so can you open an issue to remind us that we'll need to add an example once it is merged?

Originally posted by @emmanuelle in #3544 (comment)

[jni]

@seanbudd Wow, hello! 😊 How are you doing? And would you happen to be looking for some open source freelancing work? Let me know if so! 😊

Anyway, this is not blocked by #3544 because I pulled out a few of the bits in #3544 into smaller PRs, including NMI. https://scikit-image.org/docs/stable/api/skimage.metrics.html#normalized-mutual-information

[seanbudd]

@jni Hello! doing well, hope you are too. As much as I really appreciate the offer, working full time for nvaccess/nvda has my work cut out for me.

I'm hoping to progress #3544 slowly where I can in my spare time.

[jni]

Cool! It was worth a shot. 😃

[github-actions]

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