docs: add Zero sync documentation and update existing docs

- Create how-to/zero-sync.mdx setup guide
- Update docker-compose.mdx with zero-cache service, env vars, troubleshooting
- Update dev-compose.mdx with Zero build arg
- Update install-script.mdx with zero-cache URL
- Update manual-installation.mdx with Zero frontend env var
- Add Zero sync page to how-to index, meta.json, and sitemap

surfsense_web/content/docs/how-to/index.mdx

@@ -8,6 +8,11 @@ import { Card, Cards } from 'fumadocs-ui/components/card';
 Practical guides to help you get the most out of SurfSense.
 
 <Cards>
+  <Card
+    title="Real-Time Sync with Zero"
+    description="How SurfSense uses Rocicorp Zero for instant real-time data synchronization"
+    href="/docs/how-to/zero-sync"
+  />
   <Card
     title="Realtime Collaboration"
     description="Invite teammates, share chats, and collaborate in realtime"