[Doc] Refine the README for prompt_regitstry

[joshuayao]

Priority

P1-Stopper

OS type

Ubuntu

Hardware type

Xeon-GNR

Running nodes

Single Node

Description

Please read the guidelines in #1398.

If this task deprecates the component in v1.4, we can skip this task.

[CICD-at-OPEA]

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