Extend validator guide with concrete validations a beacon node should perform for builder blocks

A builder provides an `ExecutionPayloadHeader` when offering bids to a given proposer.

The proposer (and specifically the local beacon node the proposer is running) should check the header against their local execution state to ensure the header is not inaccurate or even malicious.

We should specify the set of validations that should be performed in the validator guide.

Start by locating the validator guide and the specification definitions for ExecutionPayloadHeader. Review how the proposer’s local execution state is described, then document the concrete header validations expected of a beacon node. Done means the validator guide clearly specifies the complete validation set and its scope.

Written by the indexing model from the issue text.