Clarify Step 1 in Custom Domains documentation

What does this MR do?

While assisting a customer who had questions about adding custom domains, I noticed that step 1 on the instructions didn't actually say to enter the domain in the text field. The optional SSL portion of step 1 was also a bit unclear. This MR attempts to clarify step 1 by adding an instruction to input the domain name, and making the optional step more explicit.

Related issues

No issue, but the customer's ticket is here (internal link)

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