Add alert to disable Cloudflare proxy when setting a custom domain on the cloud

[FelixMalfait]

Somehow custom domains on the cloud suddenly stopped working for people who had the Cloudflare Proxy enabled.

We need to figure out why (looks like a change on Cloudflare's side? But would be nice to confirm), and probably an alert in the dedicated page, or maybe better would be to create a dedicated help page that guides on step-by-step custom domain setup in the help center and link to it from the app (we now have a helper to link to help center articles in the right language)