Add new file

doc/api/subnet6-list.json

+{
+    "name": "subnet6-list",
+    "brief": "This command is used to list all currently configured subnets. The subnets are returned in a brief form, i.e. a subnet identifier and subnet prefix is included for each subnet.",
+    "description": "See <xref linkend=\"idp66\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.3.0",
+    "hook": "subnet_cmds",
+    "cmd-syntax": "{
+    \"command\": \"subnet6-list\"
+}",
+    "resp-syntax": "{
+    \"result\": 0,
+    \"text\": \"2 IPv6 subnets found\",
+    \"arguments\": {
+    \"subnets\": [
+        {
+            \"id\": 11,
+            \"subnet\": \"2001:db8:1::/64\"
+        },
+        {
+            \"id\": 233,
+            \"subnet\": \"3000::/16\"
+        }
+    ]
+}",
+    "resp-comment": "If no IPv6 subnets are found, an error code is returned along with the error description."
+}
\ No newline at end of file