docs(node): Engine interactions + sync strategy

[clabby]

Overview

So far, the engine API interactions that the kona-node must perform to kick off EL sync between various OP Stack execution clients has been non-intuitive, and a lot of reverse engineering + parsing of an ambiguous L1 engine API spec has been required to get it working.

Once the sync strategy and engine interactions have landed in a stable spot, we should document how this process works to alleviate the burden of onboarding contributors and give a good reference to other people that look to implement the rollup node.