Simplify removal of next/prev buttons

As documented on how to override template blocks:

https://www.mkdocs.org/user-guide/customizing-your-theme/#overriding-template-blocks

This is a big diff. I tested it by generating docs before and after the
change, and they were functionally identical.

docs/overrides/main.html

@@ -0,0 +1,4 @@
+{% extends "base.html" %}
+<!-- Get rid of the next/prev buttons -->
+{% block next_prev %}
+{% endblock %}