Add ZFS native encryption section to the handbook

[bcr]

Summary

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, zfs unload-key). Then 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.

Test Plan

1. apply the patch to the handbook
2. View the output (new section 23.6)
3. Comment or approve

[bcr]

Update the diff with the proper full path within the doc repo.

[bcr]

And now in git-format-patch(1) style.

[bcr]

... and now with context (sorry for the churn).

[bcr]

Accidentally closed, please still review.

[bcr]

Add some ZFS experts for their opinion. Thanks in advance!