[Power BI] Add comprehensive documentation for configuring auto refresh in Power BI service

[Copilot]

This PR adds comprehensive documentation for configuring scheduled refresh in Power BI service for FinOps toolkit reports, addressing the issue raised by @simonarbel who had difficulty finding guidance on this topic.

Changes Made

New Documentation Section

Added a complete "Configure scheduled refresh in Power BI service" section to setup.md that includes:

• Clear guidance on when refresh is needed: Explains which data sources (storage-based reports, Azure Resource Graph, FinOps toolkit open data) require scheduled refresh vs. those that don't (FinOps hubs with Azure Data Explorer/Microsoft Fabric)
• Step-by-step setup instructions: Detailed walkthrough covering publishing reports, configuring semantic model refresh, setting up data source credentials, and configuring scheduled refresh
• Refresh frequency recommendations: Best practices for different types of data (daily for cost data, weekly for resource data, etc.)
• Troubleshooting guidance: Common issues and solutions for refresh failures
• Links to official documentation: References to Microsoft's Power BI and Fabric refresh documentation

Cross-Reference Updates

• Updated help-me-choose.md to reference the new auto refresh documentation in the incremental refresh footnote
• Updated reports.md to include a reference to the scheduled refresh documentation after the setup instructions
• Updated changelog.md to document this new feature under Power BI reports v12

Benefits

This documentation will help users:

• Understand when they need to configure refresh vs. when it's automatic
• Successfully set up scheduled refresh without trial and error
• Troubleshoot common refresh issues
• Choose appropriate refresh frequencies for their use case

The solution directly addresses the feedback from the community discussion and provides the missing documentation that users have been requesting.

Fixes #1298.

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