Add an example of pip install --group (#12189)

This is another example for the build customization example gallery,
demonstrating how to use `pip` to install from a dependency group.


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:books: Documentation previews :books:

- User's documentation (`docs`):
https://docs--12189.org.readthedocs.build/12189/

<!-- readthedocs-preview docs end -->

<!-- readthedocs-preview dev start -->
- Developer's documentation (`dev`):
https://dev--12189.org.readthedocs.build/12189/

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Author: sirosen

docs/user/build-customization.rst

@@ -486,6 +486,32 @@ If a ``uv.lock`` file exists it is respected.
          install:
             - UV_PROJECT_ENVIRONMENT="${READTHEDOCS_VIRTUALENV_PATH}" uv sync --frozen --group docs
 
+Install dependencies from Dependency Groups
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Python `Dependency Groups <https://packaging.python.org/en/latest/specifications/dependency-groups/>`_
+are a way of storing lists of dependencies in your ``pyproject.toml``.
+
+``pip`` version 25.1 and later, which is the default for Read the Docs builds,
+as well as many other tools support Dependency Groups.
+This example uses ``pip`` and installs from a group named ``docs``:
+
+.. code-block:: yaml
+   :caption: .readthedocs.yaml
+
+   version: 2
+
+   build:
+      os: ubuntu-24.04
+      tools:
+         python: "3.13"
+      jobs:
+         install:
+            - pip install --group 'docs'
+
+For more information on relevant ``pip`` usage, see the
+`pip user guide on Dependency Groups <https://pip.pypa.io/en/stable/user_guide/#dependency-groups>`_.
+
 Update Conda version
 ~~~~~~~~~~~~~~~~~~~~