[IMP] contributing: flatten hierarchy of "documentation" pages

The content of the introduction guide is moved to the parent page which
is itself renamed to "Write documentation". The homepage's quick links
are adapted accordingly.

The chance is also taken to move media files in the appropriate folder.

task-2558998

extensions/odoo_theme/layout_templates/homepage.html

@@ -122,8 +122,13 @@
                 <h5 class="text-muted pt-3 text-uppercase fw_semibold">Top Links</h5>
                 <ul class="list-unstyled">
                     <li>
-                        <a href="{{ pathto('contributing/documentation/introduction_guide') }}" class="stretched-link">
-                            Introduction guide
+                        <a href="{{ pathto('contributing/documentation') }}" class="stretched-link">
+                            Write documentation
+                        </a>
+                    </li>
+                    <li>
+                        <a href="{{ pathto('contributing/documentation/content_guidelines') }}" class="stretched-link">
+                            Content guidelines
                         </a>
                     </li>
                 </ul>