Docs: explain go templates in custom commands

[jchvz]

This PR edits the Config.md doc to explain the go template variables that are available to authors of custom commands.

With this info documented, it becomes much easier for users to write commands such as:

customCommands:
  containers:
    -
      name: dive
      attach: true
      command: dive {{ .Container.Container.ImageID }}
      serviceNames: []

This also takes care of a minor yaml issue in the doc. Before the change, Github highlighted this as invalid:

[jesseduffield]

Thanks for making this @jchvz !