docs: add configuration reference

[blindzero]

IdLE is primarily configured via declarative configuration (workflows, steps, providers, conditions, parameters).
Currently, there is no single, authoritative reference describing the configuration structure, keys, defaults, and semantics.

This issue introduces a Configuration Reference documenting the supported configuration model.

Scope

• Document the configuration structure (root sections)
• Describe required vs optional keys
• Explain defaults and validation behavior
• Provide minimal and realistic examples
• Reference common error scenarios

Out of scope

• No automatic schema generation (future work)
• No validation logic changes

Acceptance Criteria

• A new document exists under docs/reference/configuration.md. The document includes:
  • Root structure overview
  • Step configuration examples
  • Provider configuration examples
  • Conditions / skip semantics
• Linked from docs/index.md
• No runtime code changes