docs: move documentation to docs.picpeak.app, drop in-repo copies

The full documentation now lives at https://docs.picpeak.app — built
from the picpeak-docs Nextra repo. The README, SIMPLE_SETUP, and the
v1 OpenAPI generation flow all point there now.

Removed (now living at docs.picpeak.app):
- DEPLOYMENT_GUIDE.md (root-level — content covered by docs.picpeak.app/deployment)
- docs/ADMIN_SETUP_GUIDE.md
- docs/JWT_SECRET_MIGRATION.md
- docs/SECURITY_BEST_PRACTICES.md
- docs/admin-api-quickstart.md → docs.picpeak.app/api
- docs/nginx-fix.md → docs.picpeak.app/deployment/reverse-proxy
- docs/openapi.json, docs/openapi.yaml → still generated locally as a
  build artifact (now gitignored), synced into picpeak-docs by
  scripts/sync-api-docs.sh
- docs/picpeak-admin-api.openapi.yaml → ditto

Kept:
- docs/*.png (logo + screenshots — README still img-tags these)

Updated:
- README.md — replaced six in-repo doc links with docs.picpeak.app
  pointers, restructured the Documentation section as a curated link
  list to the new site
- SIMPLE_SETUP.md — single deployment-guide link redirected
- .gitignore — docs/openapi.{json,yaml} are now build artifacts, not
  tracked
- backend/src/routes/v1/events.js — comment clarifies the OpenAPI flow

SIMPLE_SETUP.md

@@ -468,8 +468,8 @@ sudo -u picpeak node scripts/reset-admin-password.js
    - Installation: `/tmp/picpeak-setup-*.log`
 
 2. **Documentation:**
-   - [Full Documentation](https://github.com/the-luap/picpeak)
-   - [Deployment Guide](./DEPLOYMENT_GUIDE.md)
+   - [Full Documentation](https://docs.picpeak.app)
+   - [Deployment Guide](https://docs.picpeak.app/deployment)
 
 3. **Support:**
    - [GitHub Issues](https://github.com/the-luap/picpeak/issues)