Update documentation page route key formatting to use normalized parent

This should generally have the same semantics, however, it allows us to use the numbering prefix normalization provided by #1709

packages/framework/src/Pages/DocumentationPage.php

@@ -70,7 +70,7 @@ public function getTableOfContents(): string
     public function getRouteKey(): string
     {
         return Config::getBool('docs.flattened_output_paths', true)
-            ? unslash(static::outputDirectory().'/'.basename($this->identifier))
+            ? unslash(static::outputDirectory().'/'.basename(parent::getRouteKey()))
             : parent::getRouteKey();
     }