Replace taxon-list with document-list gem component

- Use the `document-list` component from the gem instead of the `taxon-list` app component in the `tagged_content_list.html.erb` and `accordion.html.erb` view templates
- By using the document-list component, the list will change from an ordered list to an unordered list, this feels like a slight improvement as the content presented does not appear to be in order of sequence/importance
- `heading_level` variable removed from the `tagged_content_list.html.erb` template, the document-list component does not support headings and is not required here

This approach is also consistent with other places the document-list component is used, for example:

- [Education, training and skills - GOV.UK](https://www.gov.uk/education)  (collections)
- [Find contact details for services - GOV.UK](https://www.gov.uk/contact)  (feedback)
- [Travel or do business in Europe: Brexit guidance](https://www.gov.uk/government/collections/brexit-guidance) (government-frontend)

app/views/world_wide_taxons/_tagged_content_list.html.erb

@@ -1,10 +1,9 @@
 <% if presented_taxon.tagged_content.any? %>
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-two-thirds">
-      <% heading_level = heading_level ? heading_level : false %>
-      <%= render 'components/taxon_list', {
-          heading_level: heading_level,
-          items: taxon_list_params(presented_taxon)
+      <%= render "govuk_publishing_components/components/document_list", {
+        remove_top_border: true,
+        items: taxon_list_params(presented_taxon)
       } %>
     </div>
   </div>

app/views/world_wide_taxons/accordion.html.erb

@@ -17,10 +17,10 @@
         %>
         <% presented_taxon.accordion_content.each_with_index do |taxon, index| %>
           <% contents = capture do %>
-            <%= render 'components/taxon_list', {
-                heading_level: 3,
-                items: taxon_list_params(taxon)
-             } %>
+            <%= render "govuk_publishing_components/components/document_list", {
+              remove_top_border: true,
+              items: taxon_list_params(taxon)
+            } %>
           <% end %>
           <%
             id = taxon.base_path || taxon.title.downcase.tr(" ","-")