CI / Local dev refresh and Helm2->3 transition

[consideRatio]

With a easy to understand CI system without tangled configuration that influence too many things at once, I hope that it will help myself and others to make contributions to this project in the long term. I wanted to update this step by step towards being easier to understand, maintain, and update it with tricks learned over time.

Work to be done

• Inline comment files on how they are used
• Collect testing config into testing folder
• Test with helm3 instead of helm2
• Create a dedicated publishing step
• Minimize rebuilds by configuring chartpress as well as possible
  • Develop the rebuildOnContextPathChanges configuration option in chartpress (Image config option added: rebuildOnContextPathChanges chartpress#98 - Review help wanted!)
  • Release chartpress 0.7.0 (Changelog for 0.7.0 chartpress#99)
  • Bump to use chartpress 0.7.0 in this repo

Review notes

• No tooling changes. We still use minikube and I have not introduced new tooling (except the helm v2 to v3 migration).
• No changes to BinderHub or Helm chart itself. This PR only impacts the CI system local development instructions, and the guide with regards to now having instructions on how to use Helm 3 rather than Helm 2.
• CONTRIBUTING.md would benefit from your review. I have improved them especially with regards to anyone wanting to develop the Helm chart in full rather than a BinderHub running locally interacting with a JupyterHub running in k8s.

Positive outcomes

1. CI refactoring reduced complexity. This will make it easier to maintain for current maintainers, but for me, the most important part is that it can attract more maintenance contributions which makes the project more sustainable in the long term.
2. Helm2 to Helm3 migration complete.
3. Helm3 based template and rendered template validation. helm3 template --validate both renders the templates but also validates them as valid k8s resources against a k8s api-server. It is a more heavy weight option to using kubeval and such tools as it requires a k8s api-server to speak with.
4. Faster CI jobs. With the chartpress feature developed, we can avoid triggering rebuilds of the BinderHub docker image for the Helm chart all the time, which can save us multiple minutes in build time.

[sgibson91]

Thank you for this work @consideRatio! ✨ I have just left a few comments but this looks good to me :)

[consideRatio]

@sgibson91 ❤️ 🎉 thank you for the review!!

[consideRatio]

I am not looking to many any more changes to this PR at this point, and it is ready for a final review.

Merging this gives us faster CI + faster upgrades by not replacing the docker image as often for no reason.

[minrk]

Looks nice. Thanks!

[consideRatio]

Chartpress 0.7.0 ability to not trigger a build of the images when not needed demonstrated to work in the CI system now.