zfsconcepts: add description of block cloning

zfsconcepts: add description of block cloning

Here I'm trying to succinctly introduce the concept, the basics of its
construction, how its different to dedup, how to use it, and where its
limitations lie, in four paragraphs and with enough searchable terms to
help the reader find more information both within OpenZFS and elsewhere.

Phew.

Sponsored-By: Klara, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #15362