[Docs] Document how Skaffold injects a default deployer

[renzodavid9]

Per #9134, is not clear how Skaffold injects a default deployer when there is not one configured in the skaffold.yaml file. This issue is to explain this behaviour in our docs. Some cases for it are:

• Multi module project
• No deployer defined but manifests.rawYaml defined
• No deployer defined in the config

Also, to improve UX, we should check if Skaffold itself can show a message about what's happening when it finds these cases.