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/app/sitemap.ts

@@ -212,6 +212,12 @@ export default function sitemap(): MetadataRoute.Sitemap {
 			priority: 0.8,
 		},
 		// How-to documentation
+		{
+			url: "https://www.surfsense.com/docs/how-to/zero-sync",
+			lastModified,
+			changeFrequency: "daily",
+			priority: 0.8,
+		},
 		{
 			url: "https://www.surfsense.com/docs/how-to/realtime-collaboration",
 			lastModified,