Add information about customers portal email confirmation

What does this MR do and why?

Some customers need to confirm their customers portal email address. This MR adds a section in the subscription docs and details when email confirmation is needed and provides a link to resend the confirmation email.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.