Add content to name servers page to address users who want to create child hosts

[gabydisarli]

Story

We need to add some instructional content to the name servers page to inform domain managers that if they want to create a subordinate host that can't do it in our registrar. They must contact help@get.gov for CISA analysts to do it in the admin.

Acceptance Criteria

• [ ] content is adjusted in the google doc
• [ ] content is adjusted on the name servers page

Additional Context

This is a temporary solution until we actually allow domain managers to add subordinate hosts in our registrar. To develop the content, discuss with @abroddrick or @neilmb for a technical understanding of this process.

Issue Links

Relates to: #1662

[abroddrick]

Just a note that can be discussed when tackling this ticket: Subordinate host is being used here for a hostname that belongs to another .gov domain. This ticket should be for messaging regarding adding a subordinate host that doesn't already exist as our system. For example, "ns.mydomain.gov" can be added to either mydomain.gov or example.gov just fine if ns.mydomain.gov was already created in our system by mydomain.gov. If mydomain.gov hasn't created that host name yet, then example.gov will see an error when attempting to use it ( You can't use it if it doesn't exist yet)

[PaulKuykendall]

This will be revisited as part of DNS Hosting development effort.