[nnx] add demo.ipynb

[cgarciae]

What does this PR do?

Adds an NNX demo notebook.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d9585e0) 56.14% compared to head (385e234) 56.13%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3680      +/-   ##
==========================================
- Coverage   56.14%   56.13%   -0.01%     
==========================================
  Files         102      102              
  Lines       12186    12220      +34     
==========================================
+ Hits         6842     6860      +18     
- Misses       5344     5360      +16     

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[chiamp]

Maybe consider executing the code blocks, so users can see the expected output of the print statements.

For reference, this is the rendered notebook that I see.