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.
We need to add a sentence after the list of bullets which says:

A 'Finished' button at the bottom of the page should link to the done page<https://www.gov.uk/service-manual/user-centred-design/resources/patterns/done-pages> for the service.