Add note about announcements visibility in transparency mode setting (#11089)

## Summary of the problem
<!-- Why are these changes being made? What problem does it solve? Link
any related issues to provide more details. -->
Organizers should know how the announcement page works for
non-transparent and transparent organizations!

Closes #11087

## Describe your changes
<!-- Explain your thought process to the solution and provide a quick
summary of the changes. -->
Adds a bullet point to the list about transparency mode and a note in
the announcement overview blankslate about visibility.

<!-- If there are any visual changes, please attach images, videos, or
gifs. -->

Author: polypixeldev

app/views/events/announcement_overview.html.erb

@@ -39,6 +39,9 @@
         <p class="text-gray-500 my-1 max-w-sm">
           Create a new announcement to keep your followers up to date!
         </p>
+        <% unless @event.is_public %>
+          <p class="text-gray-500 my-1 max-w-sm">This page will not be publicly visible until you publish your first announcement.</p>
+        <% end %>
       <% else %>
         <p class="text-gray-500 my-1 max-w-sm">
           No announcements are posted yet.

app/views/events/settings/_details.html.erb

@@ -119,6 +119,7 @@
           <li>Check recipients</li>
         </ul>
       </li>
+      <li>With transparency mode off, your announcements page will only be visible once you have published your first announcement. It will always be visible with transparency mode on.</li>
     </ul>
     <div data-accordion-target="content">
       <div class="menu__divider" style="margin:0px"></div>