Document superchain contract types

[maurelian]

We need to clearly document which contracts are Proxies or not, which are singletons, which are MCP ready, etc.

This can be used as a reference: https://github.com/ethereum-optimism/optimism/blob/13d116acfa528f57082976ee1be0ed1ba98c9823/packages/contracts-bedrock/scripts/DeployImplementations.s.sol#L817-L827

It should go into https://specs.optimism.io/protocol/overview.html#core-l1-smart-contracts