Add who’s its for page

We have a policy about how suppliers are allowed to use Notify. But we
don’t explain it anywhere. Which drives contact to our support form.

This commit that adds a new page that explains the policy.

I’ve moved the related content about who else can use Notify from the
get started page to this page as well, where it doesn’t need to sit in
a details element.

tests/app/main/views/test_index.py

@@ -83,7 +83,7 @@ def test_robots(client):
     'features_letters', 'how_to_pay', 'get_started',
     'guidance_index', 'branding_and_customisation',
     'create_and_send_messages', 'edit_and_format_messages',
-    'send_files_by_email', 'upload_a_letter',
+    'send_files_by_email', 'upload_a_letter', 'who_its_for',
 ])
 def test_static_pages(
     client_request,