docs: streamline task-sdk examples, link to core tutorial

[sunank200]

• Removed duplicated examples from examples.rst.
• Added a link to the core TaskFlow tutorial (airflow-core/docs/tutorial/taskflow.rst)
• Kept only the four essential code-snippet steps in examples.rst (define DAG, write tasks, wire flow, invoke DAG)
• Removed the simplest DAG snippet and added a link to index.rst

closes: #51520

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