add provider and contract reference

[blindzero]

IdLE relies on Providers implementing specific Contracts / Capabilities (e.g. read/write identity data).
There is currently no reference explaining:

• which providers exist
• which contracts they implement
• how contracts are expected to behave

This issue adds a Provider & Contracts Reference.

Scope

• Document all built-in providers
• Document supported contracts/capabilities
• Describe expectations per contract (methods, semantics)
• Explain how Mock/File providers are used in tests

Out of scope

• No new providers
• No contract API changes

Acceptance Criteria

• A document exists under docs/reference/providers.md
• Contracts are explicitly listed and described
• Each provider lists implemented contracts
• Linked from docs/index.md