docs: close contributor gaps — Quickstart, Extending recipes, plugin install, non-Pi service - New Quickstart page: zero-hardware path to a running pet (CLI + Telegram) - New Extending shelldon page: recipes (add provider, memory op, face, scheduled job, event kind, plugin) with exact files + verify steps - Plugins: add honest 'Installing a plugin' section + current limitations (no package/enable-disable) - Running on the Pi: fix self-referential Getting Started link; document running as a service on plain Linux - Development: add a contribution loop (branch/test/gates/PR) - Architecture: glossary for AD/CAP/NFR codes - Strip internal BMAD Story/AC references from reader-facing pages; standardize test count to 700+ - Nav: add Quickstart + Extending to Home and sidebar