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 https://github.com/scikit-image/scikit-image/pull/3544#discussion_r370383042

[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[bot]]

Hey, there hasn't been any activity on this issue for more than 180 days. For now, we have marked it as "dormant" until there is some new activity. You are welcome to reach out to people by mentioning them here or on our forum if you need more feedback! If you think that this issue is no longer relevant, you may close it by yourself; otherwise, we may do it at some point (either way, it will be done manually). In any case, thank you for your contributions so far!