Improve the imagePullSecrets documentation

[consideRatio]

We are missing an example of working configuration, and could perhaps now avoid duplicating the documentation within the hub's imagePullSecret and the singleuser's imagePullSecret.

Suggested action:

• visit schema.yaml and add an example of a working configuration
• update the hub.imagePullSecret to reference the singleuser.imagePullSecret

The current generated documentation, based on the schema.yaml file:
https://zero-to-jupyterhub-with-kubernetes.readthedocs.io/en/latest/reference.html#singleuser-imagepullsecret

[consideRatio]

See @dkipping's comment as well, we should point to Azure / GKE / Amazon guides for doing this perhaps? This can be done manually by referencing the secret manually created following these guides, but no matter what, the input to the helm chart needs to follow their guides even though the helm chart can automate the creation of the secret.

#447 (comment)

[ablekh]

@consideRatio I agree with the idea of pointing to Azure (AKS) / GCP (GKE) / AWS (EKS) guides for this aspect (as well as for other platform-specific aspects). By the way, the z2jh documentation hyperlink above is incorrect; the correct one is without the "-with-kubernetes" part: https://zero-to-jupyterhub.readthedocs.io/en/latest/reference.html#singleuser-imagepullsecret.

[dkipping]

@consideRatio just referencing to this guide and mentioning that the second step (creating a kubernetes secret) will be needed to reference it in the jupyterhub's config.yaml should already help. This might e.g. be done as a note in the jupyterhub doc.