Add Docker Hub documentation to navigation

- Added sandboxes/docker-hub to the Sandboxes navigation group in docs.json
- This ensures the docker-hub.mdx file is properly accessible through the documentation navigation
- Fixes the failing check that validates all MDX files have corresponding navigation entries

Author: codegen-sh[bot]

docs/docs.json

@@ -49,7 +49,8 @@
 							"sandboxes/environment-variables",
 							"sandboxes/secrets",
 							"sandboxes/editor",
-							"sandboxes/web-preview"
+							"sandboxes/web-preview",
+							"sandboxes/docker-hub"
 						]
 					},
 					{