Add GitHub Pages workflow and improve documentation structure

[moidx]

Summary

This PR introduces automated GitHub Pages deployment for mdbook documentation and improves the overall documentation structure with better organization and Mermaid diagram support.

Changes

GitHub Pages Workflow

• Add new github_pages.yml workflow for automated mdbook documentation deployment
• Clean up docs.yml workflow by removing GitHub Pages deployment and redirect logic
• Separate concerns: docs.yml now focuses solely on Rust API documentation validation
• Use cargo xtask docs command to build mdbook documentation consistently
• Deploy from target/book directory to GitHub Pages
• Add manual workflow dispatch trigger for testing deployments

Documentation Structure

• Update SUMMARY.md to include new specification and design document sections
• Reorganize content with better categorization:
  • Specification documents (firmware resiliency, middleware, services)
  • Design documents (driver-hubris-hash, rust-trait-to-idl-conversion)
• Improve navigation structure for better user experience

Mermaid Diagram Support

• Fix Mermaid rendering issues by adding missing JavaScript dependencies
• Add mermaid.min.js and mermaid-init.js to docs/src/third_party/
• Update book.toml configuration to include Mermaid JavaScript files
• Enable proper diagram rendering in mdbook documentation

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: moidx / name: Miguel O (e8beda2, 1797278, 8ca35b6)