Update default.hbs (#3835)

Adding meta tag that will be used later by recommendations feature of devsite.

It shouldn't have any other effect.

Thanks!

Author: egilmorez

scripts/docgen/theme/layouts/default.hbs

@@ -7,6 +7,7 @@
     <meta name="project_path" value="/_project.yaml" />
     <meta name="book_path" value="/docs/reference/_book.yaml" />
     <meta name="translation" value="disabled" />
+    <meta name="page_type" value="reference" />
     <title>{{#ifCond model.name '==' project.name}}{{project.name}}{{else}}{{model.name}} | {{project.name}}{{/ifCond}}</title>
     <meta name="description" content="Reference for {{model.name}}">
     <meta name="viewport" content="width=device-width, initial-scale=1">
@@ -30,4 +31,4 @@
 {{> analytics}}
 
 </body>
-</html>
+</html>