fix: restructure artifacts directory for $ref consumption

[hdamker]

What type of PR is this?

• enhancement/feature

What this PR does / why we need it:

Phase 1 of artifacts directory restructuring (#603).

Establishes the common/ and api-templates/ directory structure so that $ref paths in templates are identical to those in API repositories:

• Moves CAMARA_common.yaml to artifacts/common/CAMARA_common.yaml
• Adds artifacts/api-templates/sample-service.yaml — a request-response CRUD template demonstrating $ref consumption of common schemas, error responses, parameters, headers, and security schemes

The sample-service template demonstrates two patterns for error responses:

• Option A (pure $ref): For responses using only generic error codes — e.g., $ref: "../common/CAMARA_common.yaml#/components/responses/Generic404"
• Option B (local definition): For responses extending generic errors with API-specific codes — e.g., ResourceNotFound404 with IDENTIFIER_NOT_FOUND alongside generic NOT_FOUND

Which issue(s) this PR fixes:

Fixes #603 (phase 1)

Does this PR introduce a breaking change?

• Yes
• No

Special notes for reviewers:

Phase 2 (after #604 merges):

• Add common/CAMARA_event_common.yaml with CloudEvent/Source/DateTime schemas
• Add notification templates replacing notification-as-cloud-event.yaml
• Add event-related API templates
• Migrate camara-cloudevents/ content to new structure

Validation results (0 errors):

Redocly bundle resolves all $ref successfully:

bundling artifacts/api-templates/sample-service.yaml...
📦 Created a bundle for artifacts/api-templates/sample-service.yaml at /tmp/bundled-sample.yaml 20ms.

Spectral lint (1 pre-existing warning from CAMARA_common.yaml openId):

 153:12  warning  camara-properties-descriptions  Property description is missing or empty:
         "openId.description" property must be truthy  components.securitySchemes.openId

✖ 1 problem (0 errors, 1 warning, 0 infos, 0 hints)

Changelog input

 release-note
fix: restructure artifacts directory — move CAMARA_common.yaml to common/, add sample-service.yaml API template with $ref consumption

Additional documentation

docs
Updated references to CAMARA_common.yaml path in README.md and CAMARA-API-Design-Guide.md

[hdamker]

Converted to draft as I have discussed with @rartych to add also the restructure of event related artifacts after
#604 is merged. Plan is to do that tomorrow.