Add new file

doc/api/network6-subnet-add.json

+{
+    "name": "network6-subnet-add",
+    "brief": "The network6-subnet-add command is used to add existing subnets to existing shared networks.",
+    "description": "See <xref linkend=\"idp77\"/>",
+    "support": [ "kea-dhcp4", "kea-dhcp6" ],
+    "avail": "1.3.0",
+    "hook": "subnet_cmds",
+    "cmd-syntax": "{
+    \"command\": \"network4-subnet-add\",
+    \"arguments\": {
+        \"name\": \"floor13\",
+        \"id\": 5
+    }
+}",
+    "cmd-comment": "The network6-subnet-add command uses exactly the same syntax for both the command and the response.",
+    "resp-syntax": "{
+    \"result\": 0,
+    \"text\": \"IPv4 subnet 10.0.0.0/8 (id 5) is now part of shared network \'floor1\'\"
+}",
+}
\ No newline at end of file