docs: align README with default execution client (Geth)

[RonCongrevekun]

Summary

The README listed Reth as the default execution client, but the actual defaults come from the root .env (CLIENT=${CLIENT:-geth}) and docker-compose.yml (${CLIENT:-geth}/Dockerfile), which both default to Geth.

Changes

• Clarify in Quick Start that the default execution client is Geth and how CLIENT overrides it.
• Update Supported Clients to list Geth as default; Reth and Nethermind as alternatives.
• Tighten the NOTE under production hardware: default image is Geth, with examples for switching clients and a pointer to reth/README.md for Reth-specific options.

Testing

• Documentation only; no code or config changes.