Switch guides to emit faq structured data

This potentially gives us more control over the presentation of the result
in external search applications.

Related to alphagov/govuk_publishing_components#1087

app/views/content_items/guide.html.erb

@@ -1,6 +1,6 @@
 <% content_for :extra_head_content do %>
   <%= machine_readable_metadata(
-    schema: :article,
+    schema: :faq,
     canonical_url: @content_item.canonical_url
   ) %>
 <% end %>

test/integration/guide_test.rb

@@ -76,4 +76,14 @@ class GuideTest < ActionDispatch::IntegrationTest
 
     assert_has_component_title(title)
   end
+
+  test "guides show the faq page schema" do
+    setup_and_visit_content_item('guide')
+
+    schema_sections = page.find_all("script[type='application/ld+json']", visible: false)
+    schemas = schema_sections.map { |section| JSON.parse(section.text(:all)) }
+
+    qa_page_schema = schemas.detect { |schema| schema["@type"] == "FAQPage" }
+    assert_equal qa_page_schema["headline"], @content_item['title']
+  end
 end