Define the public label policy for OpenGUI issues and PRs

[HarveySang]

Problem

The repository still mostly uses default GitHub labels. A small public label policy would make incoming issues easier to sort without overloading the repo with too many internal labels.

Proposed scope

Document a compact label set and when to use each label.

Acceptance criteria

• Add docs/labels.md.
• Keep the label set small and understandable to outside contributors.
• Document when to use bug, enhancement, documentation, test, security, good first issue, and help wanted.
• Avoid internal planning labels that make the issue tracker look like a project-management board.