docs: add user guide wiki pages
Add 7 new user-facing wiki pages covering:
- Getting Started (prerequisites, installation, first-time setup)
- Configuration (connection settings, SGDB API key, Steam Input, debug)
- Syncing Your Library (how sync works, platform toggles, collections, artwork)
- Managing Games (detail panel, downloads, uninstall, metadata refresh)
- BIOS Management (what BIOS files are, per-platform status, downloads)
- Save Sync (auto-sync, conflict modes, manual sync, failed retries, offline)
- Troubleshooting (common issues, fixes, Danger Zone explained)
Rewrite Home.md as a proper landing page with separate User Guide
and Technical Reference sections.