Improve Helm chart developer experience

[lambdanis]

The current state of the Helm chart devexp is the following:

• install/kubernetes/test.sh script updates the Helm chart dependencies, validates it, and generates the values reference (chart README & docs page)
• Lint helm chart CI workflow validates the Helm chart and its docs
• There is a DEVELOP.md file, but it's outdated and it says nothing about testing the Helm chart or docs

To make Helm contributions easier I suggest to refactor these bits:

• [x] Introduce install/kubernetes/Makefile, so that make -C install/kubernetes does what is currently done by test.sh, but separate tasks can be also run with separate Makefile rules. Make the docs check a Makefile rule too.
• [x] These changes will require updating the CI, release process and Renovate config too
• [ ] Remove DEVELOP.md and document the Helm development workflow in the Contribution Guide