docs: add comprehensive README documentation for Geth and Nethermind clients

[Dark-Brain07]

Summary

This PR adds documentation for the Geth and Nethermind execution clients, completing the client documentation set alongside the existing Reth README.

Changes

1. Added geth/README.md

• Overview of op-geth and its purpose as an Optimism implementation
• Cache configuration settings with defaults and descriptions
• Sync mode options (full vs snap sync)
• Network mode configuration (full vs archive)
• Exposed ports reference table
• Optional features like EthStats monitoring
• Links to RPC documentation

2. Added nethermind/README.md

• Overview of Nethermind as an enterprise-grade .NET client
• Key configuration options and environment variables
• Multi-architecture support (AMD64/ARM64)
• Exposed ports reference table
• EthStats monitoring configuration
• Snap sync with bootnode configuration
• Troubleshooting guide for common issues
• Links to official documentation

Motivation

The existing repository has a README.md for the Reth client but not for Geth or Nethermind. Adding documentation for all three supported clients:

• Improves developer experience for operators choosing a client
• Provides consistent documentation format across all clients
• Documents client-specific configuration options
• Helps new users understand the differences between clients

Testing

• Documentation review only
• Verified all configuration options match the actual entrypoint scripts
• Verified links are valid and point to correct resources

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2