Add delete image tags by reference docs for issue #40096

What does this MR do?

Added docs for issue #40096 (closed) referencing container-registry api docs.

• retrieving a token for the container registry instance wide endpoint via CI/CD
• scopes reference
• deleting tags by reference eg. CI_REGISTRY_IMAGE:CI_COMMIT_SHORT_SHA

Closes #40096 (closed)

Related issues

• #40096 (closed)

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