[Doc] Refine the README for ChatQnA

### Priority

P1-Stopper

### OS type

Ubuntu

### Hardware type

Xeon-GNR

### Running nodes

Single Node

### Description

Please read the guidelines in #1673.

Depends on https://github.com/opea-project/GenAIExamples/pull/1654

- [x] #1755
- [x] #1721
- [x] #1850
- [ ] #1857 