Revise guidance around end and done pages

* Revise guidance to previous vesions around end pages
  * This reverts commit 779abdf2034c9c412f4d76d4bfc601dfcd1f020c.
  * This reverts commit 263396b8506e4ce4afea6fb2bd16b5811e810bca.
* Add detail around done pages distinct from end pages

Contact Subhi Barakat for details on why and how these changes were made.
Two things: 
- first, we need to cancel this reversion because the currently live version is correct
- second, re contact details - let's replace the reference to "Zendesk" with a reference to the "GOV.UK Support form" (and link directly to it) once the reversion is cancelled.