Document CPU-only torch installation to speed up pip installs

pip users get CUDA torch from PyPI by default (~873 MB). Document how to
pre-install torch and torchvision from PyTorch's CPU index (~200 MB) before
installing the package, and the equivalent uv pyproject.toml configuration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: adamtheturtle

docs/source/installation.rst

@@ -6,3 +6,29 @@ Installation
    $ pip install vws-python-mock
 
 This requires Python |minimum-python-version|\+.
+
+Faster installation
+~~~~~~~~~~~~~~~~~~~
+
+This package depends on `PyTorch`_, which pip installs from PyPI as a large CUDA-enabled build (~873 MB) even on CPU-only machines.
+To get a much smaller CPU-only build (~200 MB, no CUDA dependencies), install ``torch`` and ``torchvision`` from PyTorch's CPU index before installing this package:
+
+.. code-block:: console
+
+   $ pip install torch torchvision --index-url https://download.pytorch.org/whl/cpu
+   $ pip install vws-python-mock
+
+If you manage dependencies with ``uv``, add the following to your ``pyproject.toml`` instead:
+
+.. code-block:: toml
+
+   [[tool.uv.index]]
+   name = "pytorch-cpu"
+   url = "https://download.pytorch.org/whl/cpu"
+   explicit = true
+
+   [tool.uv.sources]
+   torch = { index = "pytorch-cpu" }
+   torchvision = { index = "pytorch-cpu" }
+
+.. _PyTorch: https://pytorch.org