[Kubernetes MCPRegistry] Testing and documentation

[dmartinol]

Operator README (cmd/thv-operator/README.md)

• Update architecture diagrams
• Add configuration options
• Example MCPRegistry manifests

API Documentation (cmd/thv-registry-api/README.md)

• API endpoint documentation
• Client examples (curl)
• OpenAPI specification reference

Project Docs

• Update operator docs in https://github.com/stacklok/docs-website

Deliverables

• Fully operational and documented MVP-1

[dmartinol]

Consider this comment about using generated doc to describe the CRD configuration:
#1773 (comment)