Help: Add example for setting default CMAKE_CUDA_ARCHITECTURES value

Fixes #21302 and #21666.

Help/variable/CMAKE_CUDA_ARCHITECTURES.rst

@@ -18,3 +18,18 @@ and compiler versions.
 
 This variable is used to initialize the :prop_tgt:`CUDA_ARCHITECTURES` property
 on all targets. See the target property for additional information.
+
+Examples
+^^^^^^^^
+
+.. code-block:: cmake
+
+  cmake_minimum_required(VERSION)
+
+  if(NOT DEFINED CMAKE_CUDA_ARCHITECTURES)
+    set(CMAKE_CUDA_ARCHITECTURES 75)
+  endif()
+
+  project(example LANGUAGES CUDA)
+
+``CMAKE_CUDA_ARCHITECTURES`` will default to ``75`` unless overridden by the user.