docs: add comments explaining DID verification relationships and key purposes

[A-Chronicle]

Description

The SDK has different key purposes (ISSUING_KEY, AUTHENTICATION_KEY) used for DID verification, but new contributors (especially those learning about Self-Sovereign Identity) may not understand:

1. What are verification relationships? (authentication, assertionMethod, etc.)
2. Why do we need different key purposes?
3. When should each be used?
4. How do they relate to DID documents?

Goal

Add inline documentation/comments to key files explaining:

• DID verification relationships and their purpose
• The relationship between ISSUING_KEY and assertionMethod
• The relationship between AUTHENTICATION_KEY and authentication
• Why this matters for SSI/Web3 identity flows

Files to Document

These files are referenced in key identity operations:

• packages/lib/sdk/src/edge-agent/didFunctions/FindDIDSigningKeys.ts
• packages/lib/sdk/src/pollux/utils/jwt/CreateJwt.ts
• packages/lib/sdk/src/pollux/utils/jwt/CreateSDJWT.ts

Acceptance Criteria

• Add comments explaining verification relationships in FindDIDSigningKeys
• Explain why different purposes exist and when each is used
• Link to relevant DID/SSI standards or documentation
• Comments should be beginner-friendly for those new to DIDs

Learning Resources to Reference

• DID Core Specification
• Verification Relationships
• Self-Sovereign Identity concepts

This would be great for someone learning about DIDs and SSI!