Add PoolManager Concept doc with updated mdx formatting

[krisoshea-eth]

This PR introduces a comprehensive guide on the PoolManager to Uniswap V4 docs.

Key Features of the Document:

1. Overview of the PoolManager and its role in Uniswap V4
2. Explanation of the Singleton design pattern and its benefits
3. Detailed breakdown of the PoolManager's architecture:

• Singleton Design
• Locking Mechanism
• Pool State
• Libraries

4. Comprehensive coverage of core functionalities:

• Pool Creation
• Swaps
• Liquidity Management
• Flash Accounting

5. Introduction to the use of Transient Storage in the PoolManager

Document Structure:

• Uses MDX formatting with frontmatter
• Organized with clear headings and subheadings for easy navigation

Impact and Importance:

This document offers insights into the architectural changes from V3 to V4, particularly the shift to a Singleton design, which has significant implications for gas efficiency and protocol extensibility.

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