Clarify CUDA/CuPy install requirements in docs (#8106) (#8983)

Fixes #8106

### Description

The installation docs did not make it clear that some MONAI transforms
require CuPy for GPU-accelerated array operations. Users who ran GPU
inference hit `OptionalImportError: import cupy (No module named
'cupy')` with no obvious guidance — `pip install cupy` fails because the
real package is CUDA-version specific (`cupy-cuda12x`), and it was not
documented that the `cucim` extra provides CuPy.

This PR updates `docs/source/installation.md` to:
- Promote the GPU/CUDA setup steps into their own `## GPU-enabled
installation (CUDA and CuPy)` section (with a Table of Contents entry)
instead of an unlabeled paragraph.
- Explain that some transforms use CuPy (e.g. via `convert_to_cupy`),
what error appears when it is missing, and that CuPy is installed
through the `cucim` extra (`pip install 'monai[cucim]'`).
- Note that installing CuPy directly requires the CUDA-specific package
name (`cupy-cuda12x` / `cupy-cuda11x`), linking to the CuPy installation
guide.
- Add a cross-reference from the recommended-dependencies list.

Docs-only change; no code or public API is modified.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Lanre Shittu <136805224+Shizoqua@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>

Author: Shizoqua

docs/source/installation.md

@@ -4,6 +4,7 @@
 
 - [Installation Guide](#installation-guide)
 	- [Table of Contents](#table-of-contents)
+	- [GPU-enabled installation (CUDA and CuPy)](#gpu-enabled-installation-cuda-and-cupy)
 	- [From PyPI](#from-pypi)
 		- [Milestone release](#milestone-release)
 		- [Weekly preview release](#weekly-preview-release)
@@ -30,12 +31,36 @@ Ignite](https://pytorch.org/ignite/), please follow the instructions:
 
 - [Installing the recommended dependencies](#installing-the-recommended-dependencies)
 
-The installation commands below usually end up installing CPU variant of PyTorch. To install GPU-enabled PyTorch:
+---
+
+## GPU-enabled installation (CUDA and CuPy)
+
+The installation commands below usually end up installing the CPU variant of PyTorch. To install GPU-enabled PyTorch:
 
 1. Install the latest NVIDIA driver.
-1. Check [PyTorch Official Guide](https://pytorch.org/get-started/locally/) for the recommended CUDA versions. For Pip package, the user needs to download the CUDA manually, install it on the system, and ensure CUDA_PATH is set properly.
+1. Check the [PyTorch Official Guide](https://pytorch.org/get-started/locally/) for the recommended CUDA versions. For Pip packages, PyTorch wheels already bundle the CUDA runtime, so you only need to pick the CUDA version matching your driver from the selector and install with the provided command. You do not need to manually download CUDA or set `CUDA_PATH`.
 1. Continue to follow the guide and install PyTorch.
-1. Install MONAI using one the ways described below.
+1. Install MONAI using one of the ways described below.
+
+Installing GPU-enabled PyTorch is enough to run models and transforms on the GPU. Some transforms,
+however, additionally use [CuPy](https://cupy.dev/) for GPU-accelerated array operations (for example
+when a transform converts a CUDA tensor via `convert_to_cupy`). If CuPy is not installed, these code
+paths raise `OptionalImportError: import cupy (No module named 'cupy')`.
+
+MONAI provides a dedicated `cupy` extra that installs a compatible CuPy build:
+
+```bash
+pip install 'monai[cupy]'
+```
+
+The `cucim` extra installs [cuCIM](https://github.com/rapidsai/cucim) (`cucim-cu12` or `cucim-cu13`
+depending on your Python version), which is a separate GPU image-processing library and does not
+install CuPy.
+
+If you prefer to install CuPy directly, note that the PyPI package name is CUDA-version specific
+(e.g. `cupy-cuda12x` for CUDA 12.x, `cupy-cuda13x` for CUDA 13.x) rather than plain `cupy`. See the
+[CuPy installation guide](https://docs.cupy.dev/en/stable/install.html) for the correct package for
+your CUDA toolkit.
 
 ---
 
@@ -184,7 +209,7 @@ You can install it by running:
 ```bash
 cd MONAI/
 pip install -e .
-# or pip install -e .[all,testing] to include most of the dependencies
+# or pip install -e '.[all,testing]' to include most of the dependencies
 ```
 
 or, to build with MONAI C++/CUDA extensions and install:
@@ -212,7 +237,7 @@ $env:BUILD_MONAI="1"
 pip install -e .
 ```
 
-If the compiled extensions were built by pip against a different version of PyTorch than the one in your environment, you may need to run the above with the `--no-build-isoloation` flag to force the use of that version, or use the `--build-constraint` method.
+If the compiled extensions were built by pip against a different version of PyTorch than the one in your environment, you may need to run the above with the `--no-build-isolation` flag to force the use of that version, or use the `--build-constraint` method.
 
 To uninstall the package please run: