Change docs license to CC BY-SA 4.0

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/42891,
we're changing the docs license to be the same across CE and EE.

With that chance, let's change the Omnibus docs as well to be
consistent.

CONTRIBUTING.md

@@ -15,6 +15,10 @@ 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._
 ## Definition of done

LICENSE

@@ -200,3 +200,8 @@
    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.
+
+---
+
+All Documentation content that resides under the doc/ directory of this
+repository is licensed under Creative Commons: CC BY-SA 4.0.