docs: Make index link point to "index.html" irrespective of port.

docs/templates/layout.html

 {% extends "!layout.html" %}
 {% set css_files = css_files + ["_static/customstyle.css"] %}
+
+{# we change the master_doc variable so that links to the index
+   page are to index.html instead of <port>_index.html #}
+{% set master_doc = "index" %}