Update default.hbs (#1040)

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

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="gtm_var" data-key="docType" data-value="reference">
+    <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">