docs: add vite 6 docs to version dropdown

[LovelessCodes]

Description

This doesn't solve any issues other than you can't really get to the Vite 6 documentation, and no other alternatives have been explored. There doesn't seem to be any issues or other pull requests.

[sapphi-red]

Ah, we also need v6.vite.dev to point to a different site as it now points to the same site as vite.dev.