Show guidance on how to upload a letter

Once we’ve launched the feature we want the guidance to be discoverable.

app/main/views/sub_navigation_dictionaries.py

@@ -76,6 +76,10 @@ def using_notify_nav():
                     "name": "Send files by email",
                     "link": "main.send_files_by_email",
                 },
+                {
+                    "name": "Upload a letter",
+                    "link": "main.upload_a_letter",
+                },
             ]
         },
         {

app/templates/views/guidance/index.html

@@ -18,6 +18,7 @@
     <li><a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.edit_and_format_messages') }}">edit and format messages</a></li>
     <li><a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.branding_and_customisation') }}">add branding and customisation</a></li>
     <li><a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.send_files_by_email') }}">send files by email</a></li>
+    <li><a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('.upload_a_letter') }}">upload a letter</a></li>
   </ul>
 
   <h2 class="heading-medium">More information</h2>