handbook/zfs: Add new section about ZFS native encryption

This change adds a new section describing how to use ZFS native
encryption including examples to the handbook. It is based on my own
training material, the open-zfs.org documentation on the subject and the
corresponding man pages (zfs-load-key(8), zfs-unload-key(8)).
I ran my writeup through an AI for grammar, typo and active voice fixes,
instructing it to not make any changes to the text itself.

PR: 264531
Reviewed by: michaelo@
Event: Halifax Hackathon 202606
Differential Revision: https://reviews.freebsd.org/D57889