Add detail to CI/CD section in setup documentation

[adamnfish]

The setting up a new project documentation glosses over how to connect the CDK synth output with the CI/CD steps.

https://github.com/guardian/cdk/blob/main/docs/setting-up-a-gucdk-project.md

It would be great if it could mention actions-riff-raff, and show how that action's contentDirectories key is used to scoop up the cloudformation template produced by the CDK synth step.

Ideally it'd also mention the riff-raff.yaml file, and show how that is provided to the configPath config key of actions-riff-raff.

I think connecting all these concepts up here will help make this seem more approachable.

[adamnfish]

I'd also point to the riffraff-platform permission needing to match the project name that gets defined in the guardian/actions-riff-raff step of the CI/CD workflow.