chore(ci): disable gitbook pages workflow for fork, add healthcheck + geo env docs

gitbook-pages.yml deployed to upstream org's 9router.github.io which a fork
cannot (no GH_PAGES_DEPLOY_KEY) — renamed to .yml.disabled. Dockerfile gains
a /api/health HEALTHCHECK. .env.example documents the new pool geostrobes
env knobs (POOL_GEO_PROBE_DISABLED, GEO_PROBE_URL).

Dockerfile

@@ -49,5 +49,9 @@ RUN apk --no-cache upgrade && apk --no-cache add su-exec && \
 
 EXPOSE 20128
 
+# Health: Next serves /api/health (dashboardGuard public path).
+HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3 \
+  CMD node -e "fetch('http://127.0.0.1:20128/api/health').then(r=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
+
 ENTRYPOINT ["/entrypoint.sh"]
 CMD ["node", "custom-server.js"]