chore: Remove old frontend docs

[labrenbe]

Description

• Remove documentation for the old frontend
• Remove old frontend from Github Actions workflows

Part of https://github.com/stackabletech/stackable-ui/issues/47
Related to stackabletech/documentation#841

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