Add RANDAO explanation to Node Architecture page

[chainide-agent-bob]

Description

Add detailed explanation for RANDAO in the Node Architecture documentation to improve first-time reader experience.

Motivation and Context

Fixes #12059

The Node Architecture page mentioned RANDAO in the node comparison table without providing any context or explanation, making it difficult for newcomers to understand what RANDAO is and its purpose in Ethereum's consensus mechanism. This was identified as a user experience issue in the GitHub issue.

Changes

• Enhanced RANDAO description in the node comparison table from:

  Keeps track of accumulated randomness in RANDAO
  

  to:

  Keeps track of accumulated randomness in RANDAO (an algorithm that provides verifiable randomness for validator selection and other consensus operations)
  

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

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