[codex] Add Astro/Starlight site

[plx]

Summary

• Add a static Astro/Starlight site under site/ with the Maquette landing page converted to src/pages/index.astro.
• Add Starlight docs pages for overview, usage, configuration, and scope.
• Configure Astro for GitHub Pages at https://plx.github.io/trop/ using site: "https://plx.github.io" and base: "/trop".
• Add a GitHub Pages deployment workflow, npm Dependabot config, site ignores, README link, and navigation-guide entries.

Validation

• npm run check
• npm run build
• agentic-navigation-guide verify
• curl -I http://127.0.0.1:4323/trop/ returned 200 OK

Notes

• The Maquette design artifacts remain untracked and are not included in this PR.
• npm install reported 5 moderate npm audit findings in the Astro dependency tree.