docs: add network selection guide and provider setup examples

Add comprehensive documentation for Alloy network types and provider configuration:

- NETWORK_SELECTION.md: Guide for choosing between Ethereum, Optimism, and
  AnyNetwork types, with decision trees, chain mappings, and common mistakes
- PROVIDER_SETUP.md: Examples for rate limiting, logging, retry layers,
  provider pooling, WebSocket setup, and production-ready configurations

Author: suchapalaver