feat: add diabetes Federated Analytics example

[SdgJlbl]

Remaining TODO:

• [ ] Choose an illustration image
• [ ] amend order of examples

[RomainGoussault]

@SdgJlbl I can't find the example on the doc built by RTD: https://owkin-substra-documentation--282.com.readthedocs.build/en/282/auto_examples/index.html

[SdgJlbl]

I think we need to change the way we are structuring example folders slightly if we want to display more than one example per page 🤔

[SdgJlbl]

I'm still not totally happy with it, but I think it's in a good enough state to get a first review.