docs(core): add 1.0.0 release notes for Linker.backend change

Breaking change: Linker.backend is now a classmethod, so call sites
must use parens: Linker.backend().

Author: cpcloud

cuda_core/docs/source/release/1.0.0-notes.rst

@@ -0,0 +1,17 @@
+.. SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+.. SPDX-License-Identifier: Apache-2.0
+
+.. currentmodule:: cuda.core
+
+``cuda.core`` 1.0.0 Release Notes
+=================================
+
+
+Breaking Changes
+----------------
+
+- :meth:`Linker.backend` is now a classmethod instead of an instance property.
+  Call sites must use ``Linker.backend()`` (with parentheses) instead of
+  ``linker.backend``. This allows querying the linking backend without
+  constructing a ``Linker`` instance — for example, to choose between PTX and
+  LTOIR input before linking.