Add contextual header in generated Starlark documentation
Since our documentation structure is mostly flat, this static line might be enough for users.
RELNOTES: None.
layout: documentation
title: ${module.title}
---
+
+<a href="../concepts.html">Extensions</a> &gt;
+<a href="skylark-overview.html">API reference</a> &gt;
+${module.title}
+
<h1 id="modules.${module.name}">${module.title}</h1>
${module.documentation}
