Document new yaml based configs

Now that we are using helm, configuration is done using the network.yaml/node-defaults.yaml files. These are mostly thin wrappers around a helm values.yaml file, but it would good to have some clear documentation and examples of how to use the files.

Could also be addressed in a separate issue, but it might be worth naming the files config.yaml no matter where they are used? The user knows what they are configuring based on the directory name (currently we have networks/ and namespaces/). This would make tab complete a bit easier. There's also some UX around whether or not we need a defaults file or if we can omit it and only apply default overrides if the users provides a defaults file. This means all defaults would then be in the values.yaml file.

cc @m3dwards