Separate Helm Chart into its own repository

[tobru]

Summary

Separate the Helm Chart for K8up into its own repository to simplify the release process.

Context

Today we're having the Helm Chart for K8up in the subdirectory charts/. This brings a lot of complexity regarding the release process. It also mixes different kinds of concerns into the same repository, for example, tracking bugs or features is harder that way.

Out of Scope

• Refactor the Helm Chart

Further links

• https://github.com/k8up-io/k8up/tree/master/charts

Acceptance Criteria

• The Helm Chart is extracted into a new repository https://github.com/k8up-io/helm-chart
• The Helm repository is served from https://charts.k8up.io using GitHub pages
• When browsing https://charts.k8up.io I get presented an introductory page
• Documentation is updated accordingly

Implementation Ideas

No response

[tobru]

Feels to be more and more pressing, see confusion in #884