Add new DocIndexRetriever example

[xuechendi]

Description

Add a new example for only add data to vector store and retrieve from vector store

related AIComps change: https://github.com/opea-project/GenAIComps/pull/314

Issues

n/a.

Type of change

List the type of change like below. Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds new functionality)
• [ ] Breaking change (fix or feature that would break existing design and interface)

Dependencies

No

Tests

No