feat: Add default css framework templates

[corentinbettiol]

Since djangocms-frontend uses bootstrap 5 as it's default css framework, we may add a way to define and use bootstrap 5 templates in stories as well.

We might add a "template style" setting in stories general config, with it's default value being a list of tuples like this:

STORIES_TEMPLATE_STYLES = (
    ("Bootstrap 5", "djangocms_stories_bootstrap_5"),
    ("Default", "djangocms_stories"),
)

Then we could define a "template style" setting in stories apphook configs, which will use the Default style if not configured otherwise.

That way, we could define our own styles by creating new templates folders in our project and referencing them in the general config, and use any style in any apphook (we could have a special style for the main blog, then bootstrap 5 style used for other blogs).

What do you think of this?