docs/templates/topindex.html: Replace usage of deprecated defindex.html.

defindex.html (used by topindex.html) is deprecated, but topindex.html was
already identical other than setting the title, so just inherit directly
from layout.html.

docs/templates/topindex.html

@@ -1,4 +1,5 @@
-{% extends "defindex.html" %}
+{% extends "layout.html" %}
+{% set title = _('Overview') %}
 {% block body %}
 
   <h1>MicroPython documentation</h1>