Improve documentation for Canary deployments

We have a section under Deploy Boards for Canary deployments: https://docs.gitlab.com/ee/user/project/deploy_boards.html#canary-deployments

But it's not very clear how to actually configure anything. We should document how to use the Auto Deploy template, as well as how to configure it manually with Kubernetes. We don't need full examples, but assuming they already know how to configure k8s, what additional do they need to do to show canary deployments. We might want to show how to add canary deployments if you don't have them, but it's reasonable to consider that out of scope.

We also need to document the variables used to control the number of canary replicas. (e.g. CANARY_PRODUCTION_REPLICAS => Defaults to one)

For Auto Deploy, describe canary flow:

Merge to master => Staging => Canary => Production