Add VitePress-based static Skills Catalog with site generator and Docker deployment

[chainsaid]

Motivation

• Provide a purely static Skills Catalog that scans skills/ at build time and emits a browsable site without a backend or runtime API.
• Make the site easy to run locally and deploy via Docker/Nginx with reproducible build artifacts.

Description

• Add a site generator scripts/generate-site.mjs that scans skills/{category}/{skill}/SKILL.md and writes docs/ pages plus docs/.generated/catalog.mjs used by the theme.
• Introduce a VitePress site under docs/ including docs/.vitepress/config.mjs, a theme override (docs/.vitepress/theme/*), and generated content for index, categories, and skills pages.
• Add sample skills/ content for coding/python-debug and devops/docker-helper and the generated pages in docs/ to illustrate output.
• Add package.json scripts (generate, dev, build, preview) and a Dockerfile plus docker-compose.yml to build the site in a Node builder stage and serve it with nginx.
• Add README.md with usage instructions and .gitignore entries for node_modules and .DS_Store.

Testing

• No automated tests were run for this change.

---

Codex Task