Highlight caching recommendations on "Custom domains" docs page for Pages

[GregBrimble]

Which Cloudflare product(s) does this pertain to?

Pages

Subject Matter

Caching rules

Content Location

We have this section (https://developers.cloudflare.com/pages/platform/serving-pages/#caching-and-performance) about caching and how it shouldn't be manually configured. We should put a small warning on the "Custom domains" page (https://developers.cloudflare.com/pages/platform/custom-domains/) too, since that's where users can configure caching themselves.

It should be clear that caching should ideally not be configured on the custom domain since we handle it for them automatically already.

Additional information

No response