Install GTM snippet on Mintlify docs site (#1134)

This PR adds the Google Tag Manager snippet (GTM-T7G5S78D) to the
Mintlify documentation site as requested in CG-18613.

## Changes Made
- Added GTM script to the `head` section of `docs/docs.json`
- The script will be injected into all pages on docs.codegen.com
- Uses the provided GTM container ID: `GTM-T7G5S78D`

## Implementation Details
- Used Mintlify's `head` configuration to inject the GTM script
- The script follows the standard Google Tag Manager implementation
pattern
- No additional noscript tag needed as this is handled by Mintlify's
rendering

## Next Steps
- Once merged and deployed, Admind can verify the GTM installation is
working
- The GTM container should start receiving data from docs.codegen.com

Fixes CG-18613

---

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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');"
+		}
+	]
 }