Add nip.io domain suggestion in the CI/CD settings

Description

From https://gitlab.com/gitlab-org/gitlab-ce/issues/45561#note_69412950:

we could go one step further and show the actual <ip>.nip.io in help text next to the input so they can just copy it straight in with no effort.

Proposal

Add help text showing the domain in Settings > CI/CD details view to suggest using nip.io if you don't have a wildcard domain entry.

Domain

[input]

A domain is required to use Auto Review Apps and Auto Deploy Stages. [IP].nip.io can be used as an alternative to a custom domain. (? icon)