Fix: USD hooks leak

If hooks are not unregistered Blender will report a leak on exit.

Store the hooks as unique_ptrs to remove manual memory management and
encapsulate the previous global list inside a function. These changes
ensure that everything is cleaned up on termination.

Also makes a small change to the hook documentation for a missing
`import` statement.

Pull Request: https://projects.blender.org/blender/blender/pulls/118294

doc/python_api/examples/bpy.types.USDHook.py

@@ -55,6 +55,7 @@ import pxr.Gf as Gf
 import pxr.Sdf as Sdf
 import pxr.Usd as Usd
 import pxr.UsdShade as UsdShade
+import textwrap
 
 
 class USDHookExample(bpy.types.USDHook):