Add format examples plus numbered user/developer guide tracks

[pcharbon70]

Summary

• add two standalone format examples under examples/:
  • jido_skill-style markdown/frontmatter declaration
  • jido_command-style markdown/frontmatter declaration
• add a numbered user guide track under docs/user (basic to advanced)
• add a numbered developer guide track under docs/developer (architecture depth)
• include mermaid diagrams in developer docs where architecture/flow visualization helps
• update README.md guide links
• include new user/developer docs in mix.exs docs extras for ExDoc publishing

Notes

• no jido_skill or jido_command dependencies were added
• this is docs/examples-focused work; no runtime behavior was changed

Validation

• mix test
• result: 64 tests, 0 failures