update smart-backend-services.md

[spicyfalafel]

Note

Comprehensive rewrite of smart-backend-services.md to provide a clearer, step-by-step guide for SMART Backend Services using JWT-based client credentials.

• Adds overview, mermaid sequence diagram, and key principles for server-to-server auth
• Replaces old snippets with stepwise instructions: key generation (OpenSSL/Node), Client registration with inline jwks or jwks_uri, AccessPolicy creation, JWT construction (header, claims, signature), and token request example
• Clarifies JWKS formats and kid usage; details validation rules (iss/sub/exp/jti, RS384, signature verification)
• Streamlines FHIR API usage example and adds scopes and security considerations sections
• Updates references to SMART and RFC specs

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