Improve custom task decorator docs

[Will-thom]

Closes #33167.

This updates the custom TaskFlow decorator how-to so it reads more like a tutorial:

• explains when a provider-level @task. decorator is appropriate
• shows the decorated operator and factory function pieces separately
• documents provider.yaml as the source for task-decorators metadata in this repository
• keeps get_provider_info guidance for third-party providers

Validation:

• git diff --cached --check
• parsed the RST with docutils for basic structure; Sphinx-specific Airflow roles/directives are not available in plain docutils

[boring-cyborg]

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