chore: performing sparse search with query expansion in langchain (#443) (#453)

## Problem

Showcase the use of Pinecone sparse embeddings and sparse indexes for in
situations where dense vector search might miss specific technical
terms.

## Solution

- use Pinecone sparse embeddings
- use query expansion technique
- allow Langchain agent to search for specific technical terms from
general queries ("Which tech companies were top performers this week?"
transforms into vector search about "GOOGL performance", "NVIDIA
Performance", ...

## Type of Change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [X] None of the above: (updates to documentation)

## Test Plan

Describe specific steps for validating this change.

---------

## Problem

Describe the purpose of this change. What problem is being solved and
why?

## Solution

Describe the approach you took. Link to any relevant bugs, issues, docs,
or other resources.

## Type of Change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] This change requires a documentation update
- [ ] Infrastructure change (CI configs, etc)
- [ ] Non-code change (docs, etc)
- [ ] None of the above: (explain here)

## Test Plan

Describe specific steps for validating this change.

---------

Co-authored-by: Bogdan Buduroiu <[email protected]>

Author: jamescalam