docs(site): add Google Analytics gtag to docs entry pages

[ErickXavier]

Summary

• Add GA4 gtag snippet (G-566FC80W9J) to the of both SPA entry pages: docs/index.html and docs/404.html
• 404.html needs the tag too since it serves deep-link routes in history-mode routing

Notes

• No SRI on gtag.js by design — Google serves it dynamically, an integrity hash would break within days
• GA4 enhanced measurement tracks history-mode route changes as page views automatically
• Docs-only change, no version bump