Migrate owtf.github.io to Jekyll Pages and vendor passive scanner

[viyatb]

Summary

This PR migrates owtf.github.io from committed Next.js export artifacts to a source-first Jekyll workflow with GitHub Pages automation, while preserving the current homepage content and formatting.

What changed

• Migrated site to Jekyll templates/includes with parity-first structure:
  • _layouts/default.html
  • _includes/header.html, _includes/home-main.html, _includes/footer.html, _includes/not-found.html
  • index.html, 404.html, _config.yml, Gemfile, .ruby-version
• Moved CSS/fonts out of /_next into first-class static assets:
  • assets/css/site.css
  • assets/fonts/*
• Removed committed export artifacts:
  • /_next/**, .nojekyll, index.txt
• Added automated workflows:
  • .github/workflows/ci.yml (Jekyll build + link checks)
  • .github/workflows/deploy-pages.yml (deploy to Pages on main)
• Integrated passive scanner from upstream owtf/online-passive-scanner:
  • vendored under online-passive-scanner/
  • provenance in online-passive-scanner/UPSTREAM_SOURCE.txt
  • sync script scripts/sync-online-passive-scanner.sh
• Added migration/source docs:
  • README.md
  • docs/PARITY_CHECKLIST.md
  • docs/PASSIVE_SCANNER_SOURCE.md

Verification

• Link-target parity for homepage navigation/CTAs checked against the previous exported page.
• Local bundle exec jekyll build is not runnable in this environment due local Ruby 2.6.10; repository now declares Ruby 3.2.2 and CI uses Ruby 3.2.

Follow-ups (separate PR)

• Add scheduled passive scan workflow that publishes _data/passive_scan/latest.json.
• Add Jekyll-rendered passive scan report page.