Upload of problem metadata uses another endpoint

See: https://domjudge-org.slack.com/archives/CHHURFUM7/p1726156440653609?thread_ts=1726145487.583349&cid=CHHURFUM7
(cherry picked from commit 72b3b49)

Author: vmcj

doc/manual/import.rst

@@ -402,11 +402,11 @@ and click `Import`.
 
 To import the file using the API run the following commands::
 
-    http --check-status -b -f POST "<API_URL>/contests/<CID>/problems" [email protected]
+    http --check-status -b -f POST "<API_URL>/contests/<CID>/problems/add-data" [email protected]
 
 To import the file using the CLI run the following command::
 
-    <WEBAPP_DIR>/bin/console api:call -m POST -f data=problems.yaml contests/<CID>/problems
+    <WEBAPP_DIR>/bin/console api:call -m POST -f data=problems.yaml contests/<CID>/problems/add-data
 
 Replace ``<CID>`` with the contest ID that was returned when importing the
 contest metadata.