[doc] Add option to select the active sidebar item

If the page contains the variable "sidebaractiveurl" in the yaml
frontmatter, this is additionally used to select the active item
in the sidebar. Useful, if the page doesn't have its own item,
but wants to activate e.g. the parent page.

docs/_includes/sidebar.html

@@ -12,7 +12,7 @@
             {% if folderitem.output contains "web" %}
             {% if folderitem.external_url %}
             <li><a href="{{folderitem.external_url}}" target="_blank">{{folderitem.title}}</a></li>
-            {% elsif page.url == folderitem.url %}
+            {% elsif page.url == folderitem.url or page.sidebaractiveurl == folderitem.url %}
             <li class="active"><a href="{{folderitem.url | remove: "/"}}">{{folderitem.title}}</a></li>
             {% else %}
             <li><a href="{{folderitem.url | remove: "/"}}">{{folderitem.title}}</a></li>