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/meta.json

@@ -1,6 +1,6 @@
 {
 	"title": "How to",
-	"pages": ["realtime-collaboration", "web-search"],
+	"pages": ["zero-sync", "realtime-collaboration", "web-search"],
 	"icon": "Compass",
 	"defaultOpen": false
 }