ci: let dependabot update jupyterhub, replace JUPYTERHUB_VERSION with PIP_OVERRIDES

[consideRatio]

I wanted to make dependabot also manage the jupyterhub version. To make that happen, I had to stop pinning it explicitly here and there.

Associated with this in the PR, is a removal of the JUPYTERHUB_VERSION --build-args. I added PIP_OVERRIDES instead as it would be more generally helpful.

Action points

• Improve inline docs of chartpress.yaml so that users understand how to learn more etc if they inspect it.
• Help users make use of the new PIP_OVERRIDES argument by commenting in chartpress.yaml

[consideRatio]

I assigned this the 0.11.0 milestone as I'd like to use this opportunity to try if the automation works with a JupyterHub 1.3.0 bump which I want to be in the 0.11.0 release of the chart.

I'm also self-merging this after some self-review for a second time. I compromise on the review part to improve on the progress part.

[consideRatio]

It worked as it should, but I found that we have strict test to match the singleuser-sample version against the hub version, so we should also bump the singleuser-sample version preferably with dependabot. It probably needs to be two separate dependabot setups, but thats okay.

#1959