Add documentation for developing against upstream dependencies.

[iansuvak]

Context and scope
Documentation should reflect the flow updating dependencies in different cases. I.e. what needs to be done to pull in a new coreth / subnet-evm / avalanchego change. A diagram might be helpful

Discussion and alternatives
While at it we should also consider simplifying the logic. e.g. making GH CI E2E testing match local testing since they are now pulling dependency binaries from different places.