Rename 'End pages' to 'Done pages'

The guidance on the page refers to 'done' pages, but the title and the URL refer to 'end' (or 'confirmation') pages.
Replace reference to Zendesk with GOV.UK support form.

This is the preferred way to contact the GOV.UK team.
Update the hackpad link for 'done' pages
Introduce guidance for 'confirmation' pages

'Confirmation' pages used to be called 'end' pages but that created confusion with 'done' pages.
Merge pull request #489 from alphagov/rename-end-pages-to-done-pages

Rename 'end' pages to 'done' pages