Add Cluster Stack Operator

[jschoone]

Add Cluster Stack Operator docs as card to https://docs.scs.community/docs/category/cluster-stacks

[DEiselt]

Created SovereignCloudStack/cluster-stack-operator#245

[DEiselt]

Notes regarding content:

• "Quickstart" just points to https://github.com/SovereignCloudStack/cluster-stacks-demo currently.
• "provider-integration.md" is linked to in "Overview" and empty
• "using-local-clusterstacks.md" contains a "quickstart" which looks fitting for the Quickstart page as per Documentation File Structure