Update CODEOWNERS to use documentation group [DOC-256] (#4)

Ensure consistency across all doc repos

Fixes: [DOC-256](https://hazelcast.atlassian.net/browse/DOC-256)

[DOC-256]:
https://hazelcast.atlassian.net/browse/DOC-256?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

.github/CODEOWNERS

@@ -0,0 +1,3 @@
+# These owners will be the default owners for everything in
+# the repo
+* @hazelcast/documentation