sinkCredential model aligment in CAMARA subscriptions guide

[PedroDiez]

What type of PR is this?

• documentation

What this PR does / why we need it:

This PRs updates sinkCredential model within CAMARA Documentation.

Specifically, it updates CAMARA-API-Event-Subscription-and-Notification-Guide.md to be consistent with artifacts design, as detailed in the Issue #638.

Which issue(s) this PR fixes:

Fixes #638

Does this PR introduce a breaking change?

• Yes
• No

Special notes for reviewers:

Changelog input

 `sinkCredential model aligment in CAMARA subscriptions guide`

Additional documentation

This section can be blank.

docs

[rartych]

For consistency sample-implicit-events.yaml should be also corrected:
line 422-3:

    # Resource (response) echoes `sink` but deliberately omits `sinkCredential`
    # — credentials are never returned to the consumer.

line 478:

            `sinkCredential` is deliberately not echoed in responses.