DOC: Add real-world aggregation example to GroupBy user guide
This PR adds a real-world example using sales data to the GroupBy Aggregation section in the user guide (groupby.rst). This enhances understanding for new users by supplementing the existing animals DataFrame example with a business-style case.
The failing doc build seems unrelated to the change introduced in this PR, which only adds a real-world GroupBy aggregation example. Please let me know if any adjustments are needed—happy to update. @pandas-dev/mentors – Submitted my GSOC 2025 proposal to revamp Pandas docs!  
Would love your feedback. Ready to start work from Day 1! Thanks for the PR, but I don't think this example adds anything additional to these doc so closing. I recommend searching the issue tracker for documentation issues that have been triaged