Add Google Tag Manager snippet to Mintlify docs

- Added GTM script (GTM-T7G5S78D) to docs.json head section
- This will enable Google Analytics tracking on docs.codegen.com
- Ready for Admind to verify the installation

Author: codegen-sh[bot]

docs/docs.json

@@ -134,5 +134,11 @@
 		"posthog": {
 			"apiKey": "phc_GLxaINoQJnuyCyxDmTciQqzdKBYFVDkY7bRBO4bDdso"
 		}
-	}
+	},
+	"head": [
+		{
+			"tag": "script",
+			"content": "(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-T7G5S78D');"
+		}
+	]
 }