Change the docs license to CC BY-SA

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/42891

CONTRIBUTING.md

@@ -9,6 +9,10 @@ terms.
 
 [DCO + License](https://gitlab.com/gitlab-org/dco/blob/master/README.md)
 
+All Documentation content that resides under the [doc/ directory](/doc) of this
+repository is licensed under Creative Commons:
+[CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).
+
 _This notice should stay as the first item in the CONTRIBUTING.md file._
 
 ---

LICENSE

@@ -4,4 +4,9 @@ Permission is hereby granted, free of charge, to any person obtaining a copy of
 
 The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
 
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
\ No newline at end of file
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+---
+
+All Documentation content that resides under the doc/ directory of this
+repository is licensed under Creative Commons: CC BY-SA 4.0.