doc: add tip on creating a new zipper/nodes via `coerce`

[lread]

From @borkdude in #rewrite-clj on Slack:

$ bb -e "(require '[rewrite-clj.zip :as z]) (-> (z/of-node (rewrite-clj.node/coerce '(+ 1 2 3))) z/down z/right z/sexpr)"
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