Clarify app compatability for Nitrokey 3

[hcc23]

File: [nitrokey3/linux/index.rst] https://docs.nitrokey.com/nitrokey3/linux/index.html (well, the actual file most likely is [nitrokey3/shared/main.rst] https://docs.nitrokey.com/nitrokey3/shared/main.html)

The important attribution on the page reads "Currently the Nitrokey App can not be used for the Nitrokey 3." As this is technically correct, as a new NK3 user I interpreted this as no app supports the Nitrokey 3.... which is not completely true as the Nitrokey App 2 does have some support already.

Based on this I'd like to ask you to:

1. update the .. important:: attribution with a note about the App 2, maybe with a link to https://docs.nitrokey.com/software/nk-app2/?
2. fix a broken link to udev rules on https://docs.nitrokey.com/software/nk-app2/installation_linux

Thanks :)