Add favicon for the docs.

Author: domdfcoding

doc-source/_templates/base.html

@@ -2,4 +2,5 @@
 {% extends "!base.html" %}
 {% block extrahead %}
 	<link href="{{ pathto("_static/style.css", True) }}" rel="stylesheet" type="text/css">
+	<link rel=icon href=https://python-coincidence.github.io/assets/coincidence.ico sizes="48x48" type="image/vnd.microsoft.icon">
 {% endblock %}