chore(github): Define code owners for Groupware docs

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

Author: ChristophWurst

.github/CODEOWNERS

@@ -0,0 +1,6 @@
+/*/groupware/                                               @ChristophWurst
+/*/groupware/calendar.rst                                   @miaulalala
+/*/groupware/contacts.rst                                   @hamza221
+/*/groupware/mail.rst                                       @GretaD
+/developer_manual/digging_deeper/groupware/                 @ChristophWurst
+/developer_manual/digging_deeper/groupware/calendar.rst     @miaulalala