Add notes on what's *not* encrypted to documentation

During the presentation we were asked what was not encrypted (especially because of the words "full disk encryption", this might be unclear). Document that the following are saved on an unencrypted partition:

• Authorized keys: SSH keys of the people with access to the CryptOps initrd
• Boot partition: In contrast to some laptop setups, this does not include an encrypted boot partition. Mostly because with virtualisation enabled, it is possible that the boot images (like the kernel and initrd) are not saved on a partition that will be mounted on the booted machine eventually.