docs: Add example for aggregating percentage changes in DataTable

[RamiNoodle733]

Description

Fixes #2592

This PR adds a new documentation section showing how to properly aggregate percentage changes (like year-over-year growth rates) in DataTable total rows.

Problem

When aggregating percentage changes (e.g., growth rates), a simple mean doesn't give the correct overall growth rate because it doesn't account for the different base sizes of each row.

Solution

Use totalAgg=weightedMean with the base metric (e.g., sales, GDP) as the weightCol. This ensures the total growth rate is properly weighted by the size of each row.

Changes

• Added new "Aggregating Percentage Changes" section under Total Row examples
• Included example query and DataTable code showing proper weighted mean usage

Checklist

• For changes to the documentation website, the changes have been previewed locally
• If there are any new components, add them to the exports in src/lib/index.ts
• If there are any changes to the query store, make sure the query store types are properly reflected in TypeScript
• If there are changes to the CLI, document them in the CLI docs
• If there are changes to the VSCode extension, update the CHANGELOG