Add NPM update procedure to documentation

Documented that NPM runs in Docker at /opt/npm and added
the docker compose commands for updating it.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

homelab-documentation.md

@@ -163,6 +163,12 @@ Saved with: `netfilter-persistent save`
 - **IP:** 10.5.0.1
 - **Ports:** 80, 443, 81 (admin)
 - **SSL:** Let's Encrypt (HTTP challenge)
+- **Runtime:** Docker (docker-compose at /opt/npm)
+
+**Update NPM:**
+```bash
+cd /opt/npm && docker compose pull && docker compose up -d
+```
 
 **Proxy Hosts:**