Fix chapter titles and filenames to match TOC order

[tyfolino]

Summary

• Renames bnf-mrms-qpe-hourly.ipynb → ch4_bnf-mrms-qpe-hourly.ipynb for consistent naming
• Renames ch4_realtimeData.ipynb → ch5_realtimeData.ipynb to match its actual chapter number
• Adds missing "Chapter 4:" prefix to the BNF notebook title
• Updates myst.yml TOC to use new filenames
• Verified all cross-references between notebooks are correct

Closes #41

Test plan

• Check that the build passes
• Verify chapter URLs in the built site match the new filenames

🤖 Generated with Claude Code

[github-actions]

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[tyfolino]

The build check is timing out due to a pre-existing issue in Chapter 5 (ch5_realtimeData.ipynb), which fetches live MRMS data from AWS and renders a full CONUS interactive plot (~24.5M data points via hvplot/Bokeh). This is non-deterministic in CI — it works some runs and hangs others depending on network conditions and runner load. This is unrelated to the changes in this PR. A separate branch will address the Chapter 5 memory/execution issue.