How to override and extend basic Python Django admin templates?

To override and extend basic Python Django admin templates, we can use the extends helper.

For instance, we write

{% extends "admin:admin/index.html" %}

{% block sidebar %}
    {{block.super}}
    <div>
        <h1>Extra links</h1>
        <a href="/admin/extra/">My extra link</a>
    </div>
{% endblock %}

to use extends with the admin template file we want to extend from.

And then we add our own items below the extends block.