Update template partials example and docs for Django 6.0

[adamchainz]

Description

The existing example and docs walk through using django-template-partials. Since its functionality has been merged into the upcoming Django 6.0 (https://docs.djangoproject.com/en/6.0/releases/6.0/#template-partials), we should:

1. Update the example project to Django 6.0 and use the built-in support there.
2. Update the docs to cover how to do it on Django 6.0, now mentioning django-template-partials as a backport.

[adamchainz]

@FarhanAliRaza or @carltongibson , any appetite to help here? No worries if not.

[FarhanAliRaza]

Would love to help.

[adamchainz]

Cool. I'm talking about the docs section @carltongibson made back in #413, so you can see that PR to check what parts of django-htmx I'm talking about! I would say first steps would be to run the example project (instructions) and to build the docs (uv sync --group docs, source .venv/bin/activate, cd docs, make html).