Fix Skill Forge documentation links

[kriptoburak]

Summary

• Update the moved Gemini CLI extensions docs URL.
• Keep the LobeHub marketplace URL as plain text so automation does not fail on its rate-limited checkpoint.
• Fix template Markdown fences and footer links so generated README examples render cleanly.
• Add the missing Cross-References section for skill configuration.

Validation

• npx --yes markdown-link-check README.md docs/.md references/.md --quiet
• git diff --check

[kriptoburak]

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