docs: add key architectural comments for Base Reth Node

[NeLiKeR]

Summary

Add key architectural comments to help new Base engineers understand the node structure.

Changes

• Add module-level documentation for main node entry point
• Document Flashblocks RPC integration and real-time block updates
• Add comments for critical configuration constants
• Explain transaction tracing and mempool analysis features
• Clarify WebSocket subscription and state management logic

Impact

These comments help new developers understand the node architecture without overwhelming the codebase with excessive documentation.

[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