docs(site): add GA4 tracking and SEO metadata

[leehack]

Summary

• add global GA4 tracking for the docs site and wire the measurement ID through the Pages build workflow
• improve Docusaurus SEO defaults with structured data, sitemap settings, indexability rules, and stronger page metadata
• add a docs social card, robots.txt, and targeted descriptions for key current and versioned docs pages

Testing

• DOCS_GA_MEASUREMENT_ID=\"G-EKSZ0RZSDQ\" ./tool/docs/build_site.sh

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.23%. Comparing base (4e63edc) to head (e0aa5b8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #84   +/-   ##
=======================================
  Coverage   77.23%   77.23%           
=======================================
  Files          67       67           
  Lines        8338     8338           
=======================================
  Hits         6440     6440           
  Misses       1898     1898           

Flag	Coverage Δ
unittests	77.23% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.