docs: clarify base-consensus quick start

[tallyhuhu]

Summary

• Clarify that the default Docker Compose setup is reth with base-consensus
• Update quick start L1 settings to show BASE_NODE_L1_ETH_RPC and BASE_NODE_L1_BEACON for the default path
• Clarify that geth and nethermind use the op-node path with USE_BASE_CONSENSUS=false

Testing

• Not run; README-only change

[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 0 Sum 1