Add explanation about share and nested-neighboring

[Danix002]

As discussed in PR #392, there's a mistake induced by the programming model way of storing state: unless share/evolve are used the state is not preserved.

The mistake lies in using neighboring nested to propagate values.

This would make a great example in the explanation (for the docs) — showing:

• what the incorrect implementation looks like,
• why it fails,
• and how to fix it using share.

Suggested action:
Add a dedicated section in the tutorial/docs using this case as an educational showcase to introduce the share operator.