docs: plot_cache_mechanism.py example should have actual external data

[sylvaincom]

Which part of the documentation needs improvement?

technical_details/plot_cache_mechanism.py Sphinx example

Describe the problem found in the documentation

We have this:

However, the external data is X_test and y_test on which the EstimatorReport already previously did the predictions? This is just to simulate external data? In that case, I find it unclear for a user? cc @glemaitre

Suggested improvement

Should we have actual external data?

Additional context

No response