Add a docstring for copy-repo [RHELDST-28512]

Add docstring to the CopyRepo class for completing the docs generation.

src/pubtools/_pulp/tasks/copy_repo.py

@@ -111,6 +111,11 @@ def __call__(self, /, *args, **keywords):
 
 
 class CopyRepo(CollectorService, PulpClientService, PulpRepositoryOperation):
+    """
+    Copy content from one repository to another.
+
+    This command copies content from repository repo-A to repo-B. If the user provides a non existing repo, the command fails.
+    """
     @property
     def content_type_criteria(self):
         # Only return non-None if there were really any types given.