Document multi-chain development workflows and use cases

[kanej]

Create a document with a set of use cases of different complexity and of different kinds that reflect the need for multi-chain development flows as completely as possible.

Potential use cases include:

• L1 <-> L2 communication scenarios
• L2 <-> L2 bridging
• Integration tests for L2s
• Front end development against multiple L2s

Investigation should include the documentation of the major l2s to determine their current suggested best practices for integration tests.

We should include consideration of deployment and test fixture setup based on Ignition.