Adds 14 per-feature docs pages under docs/ covering install, quickstart,
people, notes, timeline, search, avatars, imports, vCard export, git
sync, markdown storage, doctor, and config. Replaces the placeholder
landing page with a single-page Rolodex/index-card site that fetches and
renders the docs client-side via marked + highlight.js, with code syntax
highlighting and dark-mode support. .nojekyll keeps GitHub Pages serving
the markdown files raw to the SPA.
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