chore(docs): relicense Fern stylesheet under Apache 2.0 (#2882)

Signed-off-by: Drew Newberry <anewberry@nvidia.com>

Author: drew

fern/main.css

@@ -1,13 +1,6 @@
 /*!
  * SPDX-FileCopyrightText: Copyright (c) 2023-2024 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
- * SPDX-License-Identifier: LicenseRef-NvidiaProprietary
- *
- * NVIDIA CORPORATION, its affiliates and licensors retain all intellectual
- * property and proprietary rights in and to this material, related
- * documentation and any modifications thereto. Any use, reproduction,
- * disclosure or distribution of this material and related documentation
- * without an express license agreement from NVIDIA CORPORATION or
- * its affiliates is strictly prohibited.
+ * SPDX-License-Identifier: Apache-2.0
  */
 
 /* Color themes for light and dark modes */