docs: make use of 'nightly' image for playground config

[b-long]

Just an idea, interested to see where the conversation leads 🙂

I also wonder, as a non-k8s guru, would it be reasonable to just set the value in charts/charts/platform/Chart.yaml to nightly?

charts/charts/platform/Chart.yaml

Lines 20 to 24 in 9b9c0db

	# This is the version number of the application being deployed. This version number should be
	# incremented each time you make changes to the application. Versions are not expected to
	# follow Semantic Versioning. They should reflect the version the application is using.
	# It is recommended to use it with quotes.
	appVersion: "0.1.0"

Fixes #35

[b-long]

OBE