Indexing ERC6551 Guide w/ Airstack

[YosephKS]

I'm not exactly sure, but the docs might need a new section for this, I'm open to suggestions.

For the indexing guide, it can be a simple single-page guide that covers:

• Brief explanation of indexing, what can be indexed, and common use cases
• Getting started w/ Airstack SDK + AI
• Example Queries for common use cases mentioned above
• More resources section that can link to more specific resources covered in Airstack