Backport Docs setup for the Task SDK (#51153)

[sunank200]

This PR is a continuation of #47357

To enhance the experience (i.e., rather than just being given an alphabetical list of classes/functions), I have opted not to use the AutoAPI extension's auto-generate feature, allowing me to precisely control the order and grouping of the classes.

I have organized Task-SDK API into these sections:

• Defining DAGs: DAG, dag
• Decorators: task, task_group, setup, teardown
• Bases: BaseOperator, BaseSensorOperator, BaseNotifier, BaseOperatorLink, XComArg
• Connections & Variables: Connection, Variable
• Tasks & Operators: TaskGroup, get_current_context, get_parsing_context, Param, chain, chain_linear, cross_downstream
• Assets: Asset, AssetAlias, AssetAll, AssetAny, AssetWatcher, Metadata

I also ensured that all documented references use the top-level SDK path (e.g., airflow.sdk.DAG) instead of exposing the underlying module paths (e.g., airflow.sdk.definitions.dag.DAG).

I have incorporated a test that compares the items in the generated objects.inv with the elements I re-export from airflow/sdk/__init__.py.

The airflow/sdk/__init__.pyi type stub is unfortunately necessary, as without it (and with the imports solely within the if TYPE_CHECKING block), the re-exported classes were not appearing where I wanted them. Specifically, I want users to import items directly from airflow.sdk, not the submodule.

    stubgen -o task-sdk/src -m airflow.sdk

Test this with

 breeze build-docs task-sdk

or

uv run --group docs build-docs task-sdk

Screenshots

closes: #43010
closes: #51283
(cherry picked from commit ebfc0de)

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