Add new file

doc/api/config-test.json

+{
+    "name": "config-test",
+    "brief": "The config-test command instructs the server to check whether the new configuration supplied in the command's arguments can be loaded.",
+    "description": "See <xref linkend=\"command-config-test\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.2.0",
+    "cmd-syntax": "{
+    \"command\": \"config-test\",
+    \"arguments\":  {
+        \"<server>\": {
+        },
+        \"Logging\": {
+        }
+     }
+}",
+    "cmd-comment": "where <server> is the configuration element name for a given server such as \"Dhcp4\" or \"Dhcp6\"",
+    "resp-syntax": "{\"result\": 0, \"text\": \"Configuration seems sane...\" }
+
+    or
+
+    {\"result\": 1, \"text\": \"unsupported parameter: BOGUS (<string>:16:26)\" }",
+}
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