Integrate K8up Helm chart

[ccremer]

Summary

Part 1 of #649:

• Integrates the K8up chart over from APPUiO repo (https://github.com/appuio/charts/tree/master/appuio/k8up)
• Adjustments to GH actions to support 2 kinds of different releases:
  • When releasing K8up as operator, existing tagging scheme applies, e.g. v2.3.5
  • When releasing K8up chart, git tags need to be in the format: k8up-3.0.1 (k8up as prefix, no v prefix in the version number)
  • Currently, there's no support for releasing charts as Prereleases (-rc#) yet, maybe later.
  • Release automation also attaches the K8up CRDs as a downloadable yaml, to make helm installation easier (see [k8up] Attach CRDs to GitHub release appuio/charts#419)
• Chart is bumped to v3.0.0 simply due to the move, to communicate a clear cut where the new charts are
• Charts from 2.x will not be moved and stay in the APPUiO repo for now
• Adds new labels for GitHub PRs: area:operator, area:chart, chart:k8up to help build the release notes when releasing either code or chart.
• Release process page updated in docs

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking,
  as they show up in the changelog
• Update the documentation.
• Update tests.
• Link this PR to related issues.

[Kidswiss]

LGTM