License update to CC-BY-4.0 (#120)

This PR is the result of the discussion on Nephio TSC channel regarding
the docs licensing. It was confirmed that CC-BY-4.0 license had to be
used and two things had to be changed:

1. Primary LICENSE file
2. SPDX identifier in code files

Quoting verbatim:
---Jenn Bonner (on 21st March 2024)---
Thread regarding the Docs repo incorrect license.
I've heard back from LF Legal. They've done a thorough review, and
recommend the following steps:

1. replace the Apache-2.0 license information in the Documentation repo
2. change all the SPDX identifiers for documentation int hat repo to
CC-BY-4.0 from Apache-2.0
3. Additionally, please have EasyCLA changed so that the documentation
repo is not included in EasyCLA. [DONE]
---

---------

Signed-off-by: Rahul Jadhav <nyrahul@gmail.com>

.spellcheck.yml

@@ -1,12 +1,6 @@
 ---
-# SPDX-license-identifier: Apache-2.0
-##############################################################################
-# Copyright (c) 2023 The Nephio Authors.
-# All rights reserved. This program and the accompanying materials
-# are made available under the terms of the Apache License, Version 2.0
-# which accompanies this distribution, and is available at
-# http://www.apache.org/licenses/LICENSE-2.0
-##############################################################################
+# SPDX-license-identifier: CC-BY-4.0
+# Copyright contributors to the Nephio Project
 
 matrix:
   - name: markdown