feat: Add STACK and DEMO templating parameters

[sbernauer]

Description

Part of #187

Extracted the STACK and DEMO templating out of #429, as it's the IMHO uncontroversial part. It unblocks us from adapting the demo YAMLs already (stackabletech/demos#374).
Doing so I fixed some problems regarding what to make available when, it now has the same semantics as the existing stack and demo labels. I also document them in the README.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• CRD changes approved
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)

Reviewer

• Code contains useful comments
• (Integration-)Test cases added
• Documentation added or updated
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added

[NickLarsenNZ]

Nice! LGTM