[EPIC] RKE2 docs improvements

[manuelbuil]

This issue will be the umbrella issue trying to cover the different gaps we should fill in RKE2 documentation.

Most of the improvements are related to feature gaps. These features are well documented in k3s but not in RKE2. The idea is to cover these gaps whenever it makes sense.

Moreover, having the same structure in K3s and RKE2 will allow us to easily keep things updated.

1. Upgrade with airgap images
2. Helm and Addons
3. Extend the external datastore documentation
4. New encryption key rotation
5. SELinux
6. Default Pod Security
7. Longhorn integration
8. CLI tools reorg
9. Embedded registry
10. supervisor metrics