Add new file

doc/api/subnet6-get.json

+{
+    "name": "subnet6-get",
+    "brief": "This command is used to retrieve detailed information about the specified subnet. This command usually follows the subnet6-list, which is used to discover available subnets with their respective subnet identifiers and prefixes.",
+    "description": "See <xref linkend=\"idp68\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.3.0",
+    "hook": "subnet_cmds",
+    "cmd-syntax": "{
+    \"command\": \"subnet6-get\",
+    \"arguments\": {
+        \"id\": 11
+    }
+}",
+    "resp-syntax": "{
+    \"result\": 0,
+    \"text\": \"Info about IPv6 subnet 2001:db8:1::/64 (id 11) returned\",
+    \"arguments\": {
+        \"subnets\": [
+            {
+                \"subnet\": \"2001:db8:1::/64\",
+                \"id\": 1,
+                \"option-data\": [
+                    ...
+                ]
+                ....
+            }
+        ]
+    }
+}",
+}
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