[Newsletters] [LATER] Logged-out view customization

This is a follow-up to https://github.com/Automattic/wp-calypso/issues/69752 to keep on a backburner:

It would be neat if site owners could customize the logged-out messaging - to describe why one would want to subscribe

I think this could be done with a template part.

Start by reading the linked follow-up issue #69752 and locating the newsletter logged-out view and its template-part conventions. Clarify how site owners should edit the subscription explanation and what customization boundaries are required. Done means the logged-out messaging can be customized and the expected view behavior is covered.

Written by the indexing model from the issue text.