Just the bare minimum add new text to explain the change to kubecfg
This PR should be reopened as the `jsonnet-gutted` branch, with
commentary removed and slimmed down to just code examples with as little
non-embedded commentary as possible.
This way we can consider reorganizing any pieces that would need to be
redone for example in a hypothetical Jenkins+cdk8s use case document, or
something else like Gitlab+Tanka, or Tekton+kubecfg, or any combination.
If we are going to document outside solutions and proper integrations
with them, then we should probably make their treatments as equal as
possible to avoid showing favoritism.
efb7a27a3a