documentation on root circuit & aggregation circuit

[hero78119]

We only have purely mention in https://github.com/privacy-scaling-explorations/zkevm-specs/blob/master/specs/public_inputs.md#public-input-approach for aggregation circuit, lack of quite details. For root circuit term it's even not being mentioned.
Although root/aggregation circuit is more tightly to real implementation, it will be nice in specs to have at least markdown documentation for them