Update project-level skills guide

[healer-666]

Summary

This PR updates the project-level skills guide by clarifying the source of the skills used in the first two examples.

Changes

• Add docs/skills-example/ with the two ready-to-use example skills:
  • Applicable_Fee_IDs
  • Average_Fee_Estimation
• Update docs/project-level-skills-guide.md to link the first two examples to their corresponding skill folders.
• Clarify that these example skills were generated by our internal skill generation workflow, and that the skill generation code will be open-sourced in a future release.

Notes

Only the guide-related files are included in this update.