Re: 'Public DNS' - Clarify domains and how to get a service.gov.uk

Problem with 'Public DNS' (https://hmcts.github.io/cloud-native-platform/path-to-live/public-dns.html)

https://hmcts-reform.slack.com/archives/C8SR5CAMU/p1702295668676749?thread_ts=1701881643.474869&cid=C8SR5CAMU

Should probably include a page on justice.gov.uk, apps.hmcts.net, platform.hmcts.net and service.gov.uk when to use which domain and how to get each one.