issue #297: visualize group sparsity in example group_logistic_regression

[floriankozikowski]

Context

The example plot_group_logistic_regression.py was missing a plot, so it looked empty in the doc gallery.

Contributions of the PR

This PR adds a simple bar plot to the example plot_group_logistic_regression.py to visualize group sparsity after training.
The plot shows the L2 norm of coefficients for each group, illustrating which groups were kept or removed by the regularization.

Note:
Due to the small synthetic dataset, interpretability of the resulting sparsity pattern is limited.

Checklist before merging

• The code runs correctly locally.
• The visualization is correctly generated.
• Documentation gallery should now show a figure for this example.
• Consider discussing an alternative plot (e.g. heatmap)

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Closes issue #297