DocIndexRetriever Sample Guide Needs to be written

[chickenrae]

Example can be found here: https://github.com/opea-project/GenAIExamples/tree/main/DocIndexRetriever

Documentation should follow this format: https://github.com/opea-project/docs/tree/main/examples/ChatQnA

When draft is complete please place in this folder: https://github.com/opea-project/docs/tree/main/examples

Documentation Guidelines: https://github.com/opea-project/docs/blob/main/developer-guides/doc_guidelines.rst

[haoruan]

OPEA_Team6 is working on this issue

[yinghu5]

Hi @haoruan thank you a lot for the work. please resolve the conversation in the PR. will remind reviewer do further reviewer asap.