Duplicated Kind Extension Article

[bszyman]

I found a duplicated kind extension article.

https://docs.umbraco.com/umbraco-cms/customizing/extending-overview/extension-kind

vs

https://docs.umbraco.com/umbraco-cms/customizing/extending-overview/extension-types/kind

I already have a PR open to rewrite and update the latter (#7468), therefore, we should delete the former and have it redirect to the latter.