Re-add section on getting Context in Taskflow API

[kaxil]

Was removed by mistake in #49170 and caused failures on main due to cross references

============================== apache-airflow-providers-standard ==============================
------------------------------ Error   1 --------------------
 WARNING: undefined label: 'taskflow/accessing_context_variables'

File path: /opt/airflow/providers/standard/docs/operators/bash.rst:85

  80 |             :language: python
  81 |             :dedent: 4
  82 |             :start-after: [START howto_operator_bash_template]
  83 |             :end-before: [END howto_operator_bash_template]
  84 |
> 85 | Using the ``@task.bash`` TaskFlow decorator allows you to return a formatted string and take advantage of
  86 | having all :ref:`execution context variables directly accessible to decorated tasks <taskflow/accessing_context_variables>`.
  87 |
  88 | .. exampleinclude:: /../../../airflow-core/src/airflow/example_dags/example_bash_decorator.py
  89 |     :language: python
  90 |     :dedent: 4

---

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In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in airflow-core/newsfragments.

[kaxil]

Ran locally

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[amoghrajesh]

Nice!