[Document Driven] Support navigation.redirect in _dir.yml

[Atinux]

I guess could be added around here: https://github.com/nuxt/content/blob/main/src/runtime/plugins/documentDriven.ts#L226

WDYT @farnabaz?

[farnabaz]

How do you think it should work? Does it apply only to the exact route of the directory or all the descendants?

[Atinux]

It should apply to the exact route of the directory. This will avoid having to create an empty index.md with redirect in front-matter but directly leveraging _dir.yml to specify the redirect.

[Atinux]

I think we should also support external redirects