Decide on flags or positional arguments for `gwa config set` commands

It seems unnecessary and potentially confusing that we support both `config set [key]` and `config set --[key]` (as a flag) for all the values you can set.

This results in the internal docs being oddly duplicative:

Fix:
- decide on one approach - see pros and cons in https://chatgpt.com/share/5854e6d1-efdd-42b4-89c7-3baf4edf90c5
- `oc` uses positional args for this - see https://docs.openshift.com/container-platform/4.8/cli_reference/openshift_cli/developer-cli-commands.html#oc-config-set
- `kubectl` also uses positional args (tho it is `set-[key]`) - https://kubernetes.io/docs/reference/kubectl/quick-reference/#kubectl-context-and-configuration
- If going with positional args:
- Remove the keys as available flags.
- Remove `[flags]` from example usage.