Recently added nats-js-kv needs to be mentioned in the nats service readme

[mmattel]

Post merging #8011 (Switch default registry to nats-js-kv) and after a discussion with @kobergj, we need to mention nats-js-kv in the readme of the nats service for the dev docs. That change also needs to be transported to the admin docs.

[stale]

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