Improve the docs on how to use our Helm Charts

[anna-geller]

Description

So far, there are:

• https://kestra.io/docs/installation/kubernetes

• https://kestra.io/docs/how-to-guides/kubernetes-secrets

• https://github.com/kestra-io/helm-charts/blob/master/charts/kestra/values.yaml

• https://helm.kestra.io/

• We should release the docs here https://artifacthub.io/packages/helm/kestra/kestra

• Add a step by step guide