docs: update API reference to new dataframes.bigquery.dev location (#2293)

Also:

- include link to `bigframes.bigquery.ai` in README
- add partial ordering mode recommendation to starter sample
- remove 2.0 warning

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Towards b/454350869 🦕

Author: tswast

README.rst

@@ -6,15 +6,25 @@ BigQuery DataFrames (BigFrames)
 |GA| |pypi| |versions|
 
 BigQuery DataFrames (also known as BigFrames) provides a Pythonic DataFrame
-and machine learning (ML) API powered by the BigQuery engine.
+and machine learning (ML) API powered by the BigQuery engine. It provides modules
+for many use cases, including:
 
-* `bigframes.pandas` provides a pandas API for analytics. Many workloads can be
+* `bigframes.pandas <https://dataframes.bigquery.dev/reference/api/bigframes.pandas.html>`_
+  is a pandas API for analytics. Many workloads can be
   migrated from pandas to bigframes by just changing a few imports.
-* ``bigframes.ml`` provides a scikit-learn-like API for ML.
+* `bigframes.ml <https://dataframes.bigquery.dev/reference/index.html#ml-apis>`_
+  is a scikit-learn-like API for ML.
+* `bigframes.bigquery.ai <https://dataframes.bigquery.dev/reference/api/bigframes.bigquery.ai.html>`_
+  are a collection of powerful AI methods, powered by Gemini.
 
-BigQuery DataFrames is an open-source package.
+BigQuery DataFrames is an `open-source package <https://github.com/googleapis/python-bigquery-dataframes>`_.
 
-**Version 2.0 introduces breaking changes for improved security and performance. See below for details.**
+.. |GA| image:: https://img.shields.io/badge/support-GA-gold.svg
+   :target: https://github.com/googleapis/google-cloud-python/blob/main/README.rst#general-availability
+.. |pypi| image:: https://img.shields.io/pypi/v/bigframes.svg
+   :target: https://pypi.org/project/bigframes/
+.. |versions| image:: https://img.shields.io/pypi/pyversions/bigframes.svg
+   :target: https://pypi.org/project/bigframes/
 
 Getting started with BigQuery DataFrames
 ----------------------------------------
@@ -38,7 +48,8 @@ To use BigFrames in your local development environment,
 
     import bigframes.pandas as bpd
 
-    bpd.options.bigquery.project = your_gcp_project_id
+    bpd.options.bigquery.project = your_gcp_project_id  # Optional in BQ Studio.
+    bpd.options.bigquery.ordering_mode = "partial"  # Recommended for performance.
     df = bpd.read_gbq("bigquery-public-data.usa_names.usa_1910_2013")
     print(
         df.groupby("name")
@@ -48,49 +59,16 @@ To use BigFrames in your local development environment,
         .to_pandas()
     )
 
-
 Documentation
 -------------
 
 To learn more about BigQuery DataFrames, visit these pages
 
 * `Introduction to BigQuery DataFrames (BigFrames) <https://cloud.google.com/bigquery/docs/bigquery-dataframes-introduction>`_
 * `Sample notebooks <https://github.com/googleapis/python-bigquery-dataframes/tree/main/notebooks>`_
-* `API reference <https://cloud.google.com/python/docs/reference/bigframes/latest/summary_overview>`_
+* `API reference <https://dataframes.bigquery.dev/>`_
 * `Source code (GitHub) <https://github.com/googleapis/python-bigquery-dataframes>`_
 
-⚠️ Warning: Breaking Changes in BigQuery DataFrames v2.0
---------------------------------------------------------
-
-Version 2.0 introduces breaking changes for improved security and performance. Key default behaviors have changed, including
-
-* **Large Results (>10GB):** The default value for ``allow_large_results`` has changed to ``False``.
-  Methods like ``to_pandas()`` will now fail if the query result's compressed data size exceeds 10GB,
-  unless large results are explicitly permitted.
-* **Remote Function Security:** The library no longer automatically lets the Compute Engine default service
-  account become the identity of the Cloud Run functions. If that is desired, it has to be indicated by passing
-  ``cloud_function_service_account="default"``. And network ingress now defaults to ``"internal-only"``.
-* **@remote_function Argument Passing:** Arguments other than ``input_types``, ``output_type``, and ``dataset``
-  to ``remote_function`` must now be passed using keyword syntax, as positional arguments are no longer supported.
-* **@udf Argument Passing:** Arguments ``dataset`` and ``name`` to ``udf`` are now mandatory.
-* **Endpoint Connections:** Automatic fallback to locational endpoints in certain regions is removed.
-* **LLM Updates (Gemini Integration):** Integrations now default to the ``gemini-2.0-flash-001`` model.
-  PaLM2 support has been removed; please migrate any existing PaLM2 usage to Gemini. **Note:** The current default
-  model will be removed in Version 3.0.
-
-**Important:** If you are not ready to adapt to these changes, please pin your dependency to a version less than 2.0
-(e.g., ``bigframes==1.42.0``) to avoid disruption.
-
-To learn about these changes and how to migrate to version 2.0, see the
-`updated introduction guide <https://cloud.google.com/bigquery/docs/bigquery-dataframes-introduction>`_.
-
-.. |GA| image:: https://img.shields.io/badge/support-GA-gold.svg
-   :target: https://github.com/googleapis/google-cloud-python/blob/main/README.rst#general-availability
-.. |pypi| image:: https://img.shields.io/pypi/v/bigframes.svg
-   :target: https://pypi.org/project/bigframes/
-.. |versions| image:: https://img.shields.io/pypi/pyversions/bigframes.svg
-   :target: https://pypi.org/project/bigframes/
-
 License
 -------