docs: add issuer concepts and architecture guide

New comprehensive introduction for credential issuance covering:
- Core concepts: verifiable credentials, credential formats, credential types
- VCTM (Verifiable Credential Type Metadata) explanation
- Issuer components: API Gateway, Issuer Core, Registry
- Service architecture (single process vs microservices)
- Four deployment models with decision matrix:
  - SIROS ID Hosted (SaaS)
  - Self-Hosted (On-Premise)
  - Hybrid
  - Standalone
- Security considerations: key management (software/PKCS#11), trust chain
- Mermaid diagrams throughout for visual clarity

Updates category to link to concepts as entry point.

Author: leifj

docs/sirosid/integrations/issuers/_category_.json

@@ -3,6 +3,6 @@
   "position": 2,
   "link": {
     "type": "doc",
-    "id": "sirosid/integrations/issuers/issuer"
+    "id": "sirosid/integrations/issuers/concepts"
   }
 }