Improve CNAME instructions in the "Use a custom domain" doc

[m1foley]

I found an issue with this document.

Title: Use a custom domain
Location: https://fly.io/docs/networking/custom-domain/#set-a-cname-record
Source: https://github.com/superfly/docs/blob/main/networking/custom-domain.html.markerb

Describe the issue

Following the Getting Started guide for Elixir was a great experience. After fly deploy and testing on the subsequent fly.dev domain, it was time to migrate my custom domain name. This was the first time I struggled: I use NameCheap (one of the most popular registrars) and it asks for "Host" and "Value" fields separately. The Fly instructions didn't make it clear that I needed to enter @ as the "Host", and the fly.dev domain as the "Value". The instructions assumed I knew what I was doing with DNS, but I rarely deal with DNS so I needed more clear instructions.

My suggestions:

• Be explicit about what the "Host" and "Value" fields should be.
• Include screenshots from various registrars.

Addtional info

The CNAME example in the Fly doc:

CNAME @ exemplum.fly.dev

Screenshot of what I ended up with on NameCheap: