Add TaskGroup markdown documentation support

[ytoprakc]

Description

This PR adds doc_md support for TaskGroups, allowing authors to document a TaskGroup at the same level of abstraction shown in the Dag graph and Grid views.

TaskGroups can now define markdown documentation directly or by referencing a .md file, and that documentation is serialized with the Dag, exposed through the UI structure APIs, and rendered from the TaskGroup details panel.

Changes

• Add doc_md to TaskGroup definitions and the @task_group decorator.
• Support .md file resolution for TaskGroup doc_md, matching Dag doc_md behaviour.
• Preserve explicit doc_md when a decorated TaskGroup function also has a docstring.
• Serialize and deserialize TaskGroup doc_md.
• Include TaskGroup doc_md in graph and Grid structure responses.
• Render TaskGroup documentation in the details panel using the existing markdown documentation modal.
• Add the translated modal title: Task Group Documentation.
• Update generated UI API types and schemas.
• Document TaskGroup doc_md usage.
• Add tests for SDK behavior, file resolution, serialization, and UI structure responses.

Why

TaskGroups are often the right level of abstraction for explaining a Dag to non-technical stakeholders. Before this change, documentation could be attached to a Dag or individual tasks, but not to the TaskGroup that represents a higher-level operation. This lets Dag authors provide concise summaries for grouped operations without requiring users to drill into task-level details.

• closes: Add doc_md attribute to task groups #65679

Testing

Defining a .md filepath for doc_md

@task_group(
        group_id="data_extraction",
        tooltip="This group handles data extraction from various sources.",
        doc_md="/files/docs/example.md"
    )
    def data_extraction():
        ...

---

Using docstrings when doc_md is not defined

@task_group(
        group_id="data_extraction",
        tooltip="This group handles data extraction from various sources."
    )
    def data_extraction():
        """
        # Docs for Data Extraction Task Group
        - This is an example doc from the docstrings.
        """
        ...

---

If both doc_md and docstrings are available, doc_md takes precedence.

    @task_group(
        group_id="data_extraction",
        tooltip="This group handles data extraction from various sources.",
        doc_md="""# Data Extraction Task Group\n- This is an example doc from doc_md."""
    )
    def data_extraction():
        """
        # Docs for Data Extraction Task Group
        - This is an example doc from the docstrings.
        """
        ...

---

Documentation is also available from the Grid View

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