Add comprehensive documentation for async and sync memory database connections

[Copilot]

This PR adds comprehensive usage documentation for both synchronous and asynchronous memory database connections, addressing the lack of documentation for the async features and general usage.

What's New

The README.md has been completely rewritten to provide:

Async Usage Documentation

• Complete async/await examples with proper connection management
• Concurrent programming patterns using asyncio.gather()
• Async error handling with proper resource cleanup
• Performance considerations for high-concurrency scenarios

Sync vs Async Comparison

Clear migration guide showing the differences:

# Sync API
from mem_db_utils import MemDBConnector
connector = MemDBConnector()
conn = connector.connect()
conn.set("key", "value")
result = conn.get("key")

# Async API  
from mem_db_utils.asyncio import MemDBConnector
connector = MemDBConnector()
conn = await connector.connect()
await conn.set("key", "value")
result = await conn.get("key")
await conn.aclose()  # Important!

Advanced Features

• Redis Sentinel configuration examples
• Support for multiple database types (Redis, Dragonfly, Valkey, Memcached)
• Environment variable configuration
• Custom timeout and connection options
• Concurrent worker patterns for high-performance applications

Developer Experience

• Installation and setup instructions
• Troubleshooting guide for common issues
• Real-world code examples that have been tested
• Performance guidance on when to use sync vs async

All documentation examples have been validated with real Redis instances to ensure they work correctly in practice.

Fixes #5.

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[Copilot]

Pull Request Overview

This PR significantly enhances the documentation by providing comprehensive usage examples for both synchronous and asynchronous memory database connections, addressing the previously minimal documentation that only contained a single line description.

Key changes include:

• Complete async/await documentation with proper connection management and error handling
• Sync vs async API comparison tables and migration guide
• Advanced usage patterns including concurrent operations and custom configurations