[CEP] Update configuring-custom-pages-error-and-challenge.mdx

Customers can collapse CEPs locally as well to see character size by running our collapsify tool.

Author: nenizera

src/content/docs/support/more-dashboard-apps/cloudflare-custom-pages/configuring-custom-pages-error-and-challenge.mdx

@@ -180,6 +180,7 @@ Your custom error page cannot be blank and cannot exceed 1.43 MB. To avoid excee
 
 - If you encounter errors while attempting to preview or publish your custom error page, use an [HTML validator](https://validator.w3.org/) to ensure that your code resolves properly.
 - Make sure that you are serving the custom error page with an HTTP 200 status code.
+- Collapse a custom page locally to check character size by running Cloudflare's [Collapsify tool](https://github.com/cloudflare/collapsify)  
 
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