Add Clarity tracking to every course page

[Matt Soucoup (codemillmatt)]

Summary

• Add the supplied Microsoft Clarity snippet (project ynx84zjbvb) to the shared <head> in webpage/index.html, covering every chapter and reference page.
• Document Clarity usage analytics in the site privacy notes.
• Add checks for the shared tracking script and its inclusion in the published site.

This branch starts from upstream main and includes only the three files for this change.

Validation

• npm run build
• Both targeted content tests pass: shared Clarity tracking and published page shell.
• git diff --cached --check