doc: Replace whitespaces with - for section IDs

Fixes jQuery selector issues when jumping to an anchor.

modules/doc/library/Doc/DocSection.php

@@ -88,6 +88,14 @@ class DocSection extends TreeNode
         return $this->content;
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function setId($id)
+    {
+        return parent::setId(str_replace(' ', '-', (string) $id));
+    }
+
     /**
      * Set the header level
      *