BUG: `Styler` subclassing and `from_custom_template` no longer working
PR #40312 introduced jinja2 template inheritance for the styler HTML parsing.

Essentially we went from a structure like:

```
# main.template
{% block foobar %}
<html>
{% endblock %}
```
to 

```
# main.template
{% include "sub.template" %}

# sub.template
{% block foobar %}
<html>
{% endblock %}
```

The problem with that is that Styler offers (and documents) the ability to subclass it and extend the main template so that defined blocks can be overwritten.

In jinja2 the `included` blocks of an extended template cannot be overwritten.

Hence a patch is to point the docs to extending the `sub` templates instead, and amending the `from_custom_template` method to amend each sub template instead of the main. 

This fixes all functionality but is not backwards compatible becuase of the arg change.