Add grouped error‑bar plot example to gallery

Add a demo script to galleries/examples/statistics/errorbar_features.py showing how to plot mean ± standard error for multiple groups.

Reason

Error bars are important in data analysis to communicate uncertainty. A clear grouped example helps analysts present summary statistics effectively.

Proposed Plot

• Synthetic DataFrame with groups: group, measurement.
• Compute mean and SEM per group using Pandas.
• Plot with plt.errorbar() and labeled axes.
• Add explanatory comments and optional savefig.

Benefits: Teaches group-aggregation and uncertainty visualization.