add documentation on superchain_time

[tessr]

A request that came up in our Infra Biweekly, is to publicly document the way that the superchain_time flag is set. We should get these docs added to either the SCR or the node docs.

[geoknee]

There are some docs here https://github.com/ethereum-optimism/superchain-registry/blob/main/docs/hardfork-activation-inheritance.md