Set doc_md on TaskFlow decorator example DAGs

[ColtenOuO]

Summary

Closes #60128 by setting dag.doc_md on three TaskFlow-decorator example DAGs in providers/standard/src/airflow/providers/standard/example_dags/. Without doc_md, the DAG's "Docs" tab in the UI is empty, which hurts the first-time-user / onboarding experience these examples are meant to support.

The issue body names four examples as a starting set. One of them — example_bash_decorator.py — already has a function-level docstring, which the @dag decorator auto-assigns to dag.doc_md when no explicit doc_md is set, so its Docs tab is already populated and it's intentionally left out of this PR.

The remaining three from the issue's starting set are addressed:

File	Pattern	Change
example_sensor_decorator.py	@dag decorator, only a module docstring	Added a function docstring (auto-flows into doc_md)
example_short_circuit_decorator.py	@dag decorator, only a module docstring	Added a function docstring (auto-flows into doc_md)
example_branch_operator_decorator.py	with DAG(...) block, module docstring	Restructured the existing module docstring as Markdown and passed doc_md=__doc__ to the DAG(...) call

Each doc_md follows the structure called out in the issue: H3 title, one-paragraph intro, a "What this DAG shows" bulleted list, and a link to the relevant operator/concept docs.

Was generative AI tooling used to co-author this PR?

• Yes (Claude Code) for grammar correction

Generated-by: Claude Code following the guidelines

[ColtenOuO]

The stable/operators/python.html URL on apache.org currently returns 404 in browsers due to a meta-refresh redirect to a stale archive path.

I followed the existing in-directory convention from example_bash_operator.py (no link) rather than ship dead URLs.

Happy to add links in a follow-up once the redirect is fixed.

[ColtenOuO]

The failing job Low dep tests: providers / All-prov:LowestDeps:14:3.10:mongo...samba looks unrelated to this PR — it fails with pymongo.errors.ServerSelectionTimeoutError: 172.17.0.1:32769: [Errno 111] Connection refused, i.e. the MongoDB container in CI didn't accept connections. This PR only touches three example DAG files in providers/standard/src/airflow/providers/standard/example_dags/, which have no MongoDB dependency.

Could a maintainer rerun the failed job? Happy to push an empty commit if that's preferred.