feat: Add GitHub repository link to documentation sidebar (#14496)

Author: statxc

docs/_templates/layout.html

@@ -0,0 +1,5 @@
+{% extends "!layout.html" %}
+{% block sidebartitle %}
+{{ super() }}
+<a href="https://github.com/pyca/cryptography" style="display:block;margin-top:8px;text-align:center;">View on GitHub</a>
+{% endblock %}