[DOCUMENTATION] Expand Data Corpus section

[IanHoang]

Is your feature request related to a problem? Please describe

Currently, there is no expand-data-corpus section in documentation

Describe the solution you'd like

We should add this so that users understand how to use it.

Describe alternatives you've considered

No response

Additional context

No response

[Naarcha-AWS]

Use comments found in the script to assist with documentation: https://github.com/opensearch-project/opensearch-benchmark/blob/main/scripts/expand-data-corpus.py

[Naarcha-AWS]

Lets place in the Optimizing Benchmarks section.