add PoolKey reference guide with mdx formatting

[krisoshea-eth]

This PR introduces a reference guide on the PoolKey struct in Uniswap V4.

Key Features of the Document:

1. Introduction to the PoolKey struct and its importance in Uniswap V4
2. Detailed breakdown of the PoolKey structure, including a Solidity code snippet
3. Comprehensive table describing each field in the PoolKey struct:
   • currency0
   • currency1
   • fee
   • tickSpacing
   • hooks
4. Important notes section highlighting key aspects of PoolKey usage and implementation

Document Structure:

• Uses MDX formatting with frontmatter
• Provides a Solidity code snippet for the PoolKey struct
• Includes a table for field descriptions
• Concludes with important notes for developers

Impact and Importance:

The PoolKey struct is fundamental to identifying and interacting with liquidity pools in Uniswap V4. Understanding its structure and usage is essential for anyone implementing, integrating, or building on top of Uniswap V4.

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