Add StackedBarChart example to README

[islubee]

Summary

• Adds a screenshot of StackedBarChart to the component section in README
• Updates the Full Example section to include StackedBarChart alongside the other charts
• Example data uses 6 months × 3 series (apples, oranges, bananas)

Test plan

• Screenshot renders correctly in the README on GitHub
• Full Example code snippet is complete and accurate