Simplify README and fix Pages publishing and CI status

[jrepp]

The README mixed the project introduction with detailed API documentation, repeated installation instructions, and a crowded diagram. Reduce it to 95 lines with basic C and JavaScript usage, build instructions, and a three-node vertical Mermaid diagram. Move the longer reference into a static GitHub Pages guide and link it from the README and demo.

Update CI so newer upstream dependency releases produce an advisory instead of failing the build; toolchain checks, dependency security tests, and the audit remain required. The Pages workflow now builds pull requests and deploys only from main, includes parser sources in its triggers, and uses the same Emscripten setup action as CI. Badges now track main.

Validation:

• Native CMake build and all three CTest suites passed.
• JavaScript tests, API tests, lint, and security checks passed with Node.js 26.7.0 and npm 12.0.2; npm audit reported no vulnerabilities.
• C examples compiled and ran; README and guide links and HTML anchors checked.
• Guide inspected at mobile and desktop widths in light and dark mode; Mermaid rendered in both themes.
• CI and Pages workflows passed actionlint; git diff --check passed.

The repository's Pages source has been switched to GitHub Actions. Deployment of the new guide will be verified after merge to main.