Initial implementation of custom node tutorial

[wouterwln]

Here's my initial effort to create a tutorial for making a custom node. There's a couple of things I had questions about and that I left intentionally vague, so this is by no means my final effort towards this. My questions are the following:

• When implementing this, RxInfer was really eager on using the q_out variational update rule towards p. However, in my head, there's no reason to use the variational update rule over the sum-product rule in this model
• I have absolutely no idea how the average energy calculation works, so I kind of skimmed over it.
• My explanation of how update rules work is a bit shaky, any suggestions on how to improve this? This is probably because the concept is still a bit new to me, but I didn't want to explain it using the integral-over-joint definition, although we could.

I'll expand the latter part of the explanation (from the marginal computation onwards) a bit further later, but as of writing this it's 6pm on a Friday so my brain is fried. Have a nice weekend.

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Codecov Report

Patch and project coverage have no change.

Comparison is base (20a1499) 79.91% compared to head (504e853) 79.91%.

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```diff @@ Coverage Diff @@ ## main #105 +/- ## ======================================= Coverage 79.91% 79.91% ======================================= Files 10 10 Lines 1200 1200 ======================================= Hits 959 959 Misses 241 241 ```

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[bvdmitri]

This is a great addition, I like the story and the explanation is clear in my opinion. I would, however, argue that this should be added to the documentation instead as a page (not as an example) and ReactiveMP should have a documentation page with the API documentation. Would it be too difficult to convert the example to a documentation page? The documentation does allow code execution too. See the @example macro from the Documenter.

[bvdmitri]

I've addressed my own comments. The rest is fine by me. Thanks @wouterwln and @bartvanerp !