Clarify repo skills vs legacy repo.md in SDK docs

[enyst]

Updates sdk/guides/skill.mdx to avoid implying that repo.md (or .openhands/skills/repo.md) is a canonical mechanism.

• Example Skill name changed from repo.md to grumpy-cat (name can be any identifier)
• Clarifies repo_skills are skills with trigger=None
• Clarifies knowledge_skills are triggered skills, not tied to a knowledge/ subdirectory

Context: we removed the .openhands/skills/repo.md -> AGENTS.md symlink in agent-sdk and OpenHands-CLI to prevent duplicate loading of AGENTS.md.

@enyst can click here to continue refining the PR