Helm docs need a review/rewrite

[dholbach]

In the review of #1357 it became clear that the Helm related docs need a rewrite (and/or possibly another review).

We currently have:

• chart/flux/README.md: likely in good shape
• site/helm-get-started.md: tutorial, relatively new, based on flux-helm-test (cf Consider new Helm example #1360 for the choice of example)
• site/helm-operator.md: will need review/rewrite
• site/helm-integration.md: will need review/rewrite, needs probably the most insight knowledge.

It'd be great if users of Flux+Helm could take a look at the docs above and see if they are up to date or things missing.

[stefanprodan]

The chart/flux/README.md looks good to me.

[stefanprodan]

Helm-op now has it's own docs section and Helm chart.