Update page title on publish confirmation page

Update the page title to provide an indication to the user that the page
 is not just the view page for the manual, but a page for publishing it.

Also remove the unused `:page_title` value, which was used by the old,
 legacy, layout but is not needed by the design system one.

app/views/manuals/confirm_publish.html.erb

@@ -1,7 +1,6 @@
 <% draft_sections = manual.sections.select(&:draft?) %>
 
-<% content_for :page_title, manual.title %>
-<% content_for :title, manual.title %>
+<% content_for :title, "Publish #{manual.title}" %>
 
 <% content_for :title_margin_bottom, 6 %>