Add seqproc documentation site

[rob-p]

Summary

• replace the stale MkDocs stub with an Astro Starlight documentation site
• expand installation, CLI, EFGDL, ambiguity, protocol, I/O, reporting, performance, Rust API, and troubleshooting guidance
• deploy main to combine-lab.github.io/seqproc with a path-scoped, npm-cached GitHub Pages workflow
• refresh the repository README as a concise entry point

Validation

• clean npm ci from the committed lockfile
• Astro production build: 18 static pages plus search index and sitemap
• git diff --check