Add custom sidebar content hook (#5448)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bexelbie

_includes/sidebar-custom.html

@@ -16,5 +16,6 @@
       {% include nav_list locale=locale nav=page.sidebar.nav %}
     {% endif %}
   {% endif %}
+  {% include sidebar-custom.html %}
   </div>
 {% endif %}

_includes/sidebar.html

@@ -663,6 +663,8 @@ sidebar:
 **Note:** Custom sidebar content added to a post or page's YAML Front Matter will appear below the author profile if enabled with `author_profile: true`.
 {: .notice--info}
 
+For more advanced customization, create `_includes/sidebar-custom.html` in your site. Its contents are included at the bottom of the sidebar on every page, after the author profile, YAML-defined blocks, and navigation menu. This follows the same pattern as [`head/custom.html`](#custom-head-tags) and `author-profile-custom-links.html`.
+
 ### Custom sidebar navigation menu
 
 To create a sidebar menu[^sidebar-menu] similar to the one found in the theme's documentation pages you'll need to modify a `_data` file and some YAML Front Matter.