Use nicer URLs without .html suffix on website

Also easier insertion of analytics tag for the website without
needing a custom template.

The old .html URLs are redirected via meta http-equiv refresh, which is
all gh-pages supports

docs/_templates/layout.html

@@ -1,6 +0,0 @@
-{% extends "!layout.html" %}
-
-{%- block extrahead %}
-<!-- kitty analytics placeholder -->
-  {{ super() }}
-{% endblock %}