Document explicit usages of engine api in op-node

[roninjin10]

Description

To understand the changes to the engine API and also make sure we test all uses of it are compatible we should document the specific ways op-node interops with the engine api

Related to #5

Tasks

• Document a high level view
• Document links to code locations in op-node
• Document which use cases feel like they could be left unimplemented if any
• Persist the docs in docs/engine-api.md as contributing docs and link to it in CONTRIBUTING.md
• (optional) document links to reth

[roninjin10]

@anikaraghu @zencephalon and @roninjin10 to meet with @trianglesphere about this

• Being explored actively now so should have detailed answers
• Will to gather up a list of useful links before meeting

[trianglesphere]

OP Usage & Modifications: https://github.com/ethereum-optimism/optimism/blob/develop/specs/exec-engine.md#engine-api

Paris Engine API: https://github.com/ethereum/execution-apis/blob/584905270d8ad665718058060267061ecfd79ca5/src/engine/paris.md

We use all but engine_exchangeTransitionConfigurationV1

Shanghai Engine API: https://github.com/ethereum/execution-apis/blob/584905270d8ad665718058060267061ecfd79ca5/src/engine/shanghai.md

In shanghai, we don't use the new APIs, but have updated to the V2 versions of the ones we use.

[roninjin10]

Added list of links including links @trianglesphere shared here: #66
Also @anikaraghu @zencephalon and I are chatting with @trianglesphere soon about engine api