[L2 Testnet Tutorial] Add steps on how to spin up op-challenger

[sbvegan]

Description

Now that we're using op-deployer we're deploying the latest contract versions, using PermissionedDisputeGames. This requires the op-challenger service to properly operate the chain.

Acceptance criteria

• Start op-challenger section is added to the documentation

Resources and support

• op-challenger docs are up to date
• solutions folks have access to the correct configs too

Action items

1. Add a new section to the tutorial to describe how to start the op-challenger service
2. Get a peer review
3. merge