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.
Agreed. Let's refer to the GOV.UK Support form (and link to it directly) rather than Zendesk. Let's leave out more general guidance on contacting GDS until we see that people are actually having problems requesting these pages.