GitHub pages hosted helm chart

[plumdog]

Related to #22

This shuffles the chart around so it is shaped to work with https://github.com/helm/chart-releaser-action

There is an action that runs on PRs that does the chart linting, and checks the version is bumped if the chart is changed. Then an action that runs on master that updates the gh-pages branch with the packaged chart.

I believe this would then mean there would be a chart repo at https://hiddeco.github.io/cronjobber/ so I could run:

$ helm repo add cronjobber https://helm.datadoghq.com
$ helm install my-cronjobber cronjobber/cronjobber

to install.