[docs] another 404 in docs + improve slots docs

[nerdoc]

found another one in https://django-components.github.io/django-components/latest/reference/template_variables/#django_components.component.ComponentVars.is_filled

the "Use component_vars.slots instead" is a 404.

[nerdoc]

Hm, maybe when fixing this, it could be also explained better. I think the link should go to https://django-components.github.io/django-components/latest/concepts/fundamentals/render_api/#slots - but I am not sure, as there is no explanation of the usage like "header" in slots This is only explained as "also possible" in the is_filled doc. I think it should be made clear too in the "slots" doc.

[EmilStenstrom]

I think there should probably be a validation script that looks for 404s before publishing.

[nerdoc]

Hm... https://pypi.org/project/mkdocs-linkcheck/ ? This only works with a pre-commit - better would be a plugin for mkdocs... that prints the errors in STDOUT, like other errors...

[JuroOravec]

The issue of link validation is captured in https://github.com/django-components/django-components/issues/1343. Let's keep this issue focused on the broken link and this:

Hm, maybe when fixing this, it could be also explained better. I think the link should go to django-components.github.io/django-components/latest/concepts/fundamentals/render_api#slots - but I am not sure, as there is no explanation of the usage like "header" in slots This is only explained as "also possible" in the is_filled doc. I think it should be made clear too in the "slots" doc.