[ScottishTapWater]

Summary

• Put requirements and the first calculation before the package catalogue.
• Group quantity operations, unit conversions, generator behaviour, diagnostics, and extension authoring into clear sections.
• Remove the obsolete combined-package migration note.
• Add a small select job that always checks changed paths and PR-title validity. The required build job runs for changes under src/ or .github/workflows/ci.yml, and also fails if selection or title validation fails. Manual CI runs still execute the full pipeline.

Verification

• git diff --check passed.
• Confirmed all 14 code examples and 10 external links are preserved.
• Independent review found no actionable issues.

No local .NET tests were run; this PR changes documentation and workflow configuration. Because it changes ci.yml, the full CI pipeline runs on the PR.