Update license to CC BY 4.0 (dotnet#1381)

* Update license to CC BY 4.0

* Update license.txt to match core-docs

Author: danroth27

LICENSE.txt

@@ -1,12 +1,10 @@
-Copyright (c) .NET Foundation. All rights reserved.
+Copyright (c) .NET Foundation and contributors. All rights reserved.
 
-Licensed under the Apache License, Version 2.0 (the "License"); you may not use
-these files except in compliance with the License. You may obtain a copy of the
-License at
+Documentation is licensed under the Creative Commons Attribution 4.0
+International License (http://creativecommons.org/licenses/by/4.0/legalcode).
 
-http://www.apache.org/licenses/LICENSE-2.0
+Code is licensed under the MIT License (https://opensource.org/licenses/MIT).
 
-Unless required by applicable law or agreed to in writing, software distributed
-under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
-CONDITIONS OF ANY KIND, either express or implied. See the License for the
-specific language governing permissions and limitations under the License.
+This license does not grant you rights to use any trademarks or logos of
+Microsoft. For Microsoft’s general trademark guidelines, go to
+http://go.microsoft.com/fwlink/?LinkID=254653.

common/_templates/footer.html

@@ -1,5 +1,5 @@
 {% extends "!footer.html" %}
 
 {%- block extrafooter %}
-  Documentation licensed under <a href="https://creativecommons.org/licenses/by-nc/3.0/">CC BY-NC 3.0</a>.
+  Documentation licensed under <a href="https://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a>.
 {% endblock %}