docs: Put the v2 migration guide in a Migration guides toctree (#3387)

module/v2_migration was not in any toctree, which produced a Sphinx
"document isn't included in any toctree" warning. Add it to its own
"Migration guides" section at the bottom of the API reference, and
rename the module toctree caption from the "CaptionHolder:" placeholder
to "Modules:".

Part of #3383.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

Author: mdboom

cuda_bindings_13/docs/source/api.rst

@@ -7,7 +7,7 @@ CUDA Python API Reference
 
 .. toctree::
    :maxdepth: 3
-   :caption: CaptionHolder:
+   :caption: Modules:
 
    module/driver
    module/driver_v2
@@ -21,3 +21,9 @@ CUDA Python API Reference
    module/cufile
    module/nvml
    module/utils
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Migration guides:
+
+   module/v2_migration