Update documentations

docs/configuration/outbound/dns.md

@@ -0,0 +1,22 @@
+`dns` outbound is a DNS server.
+
+### Structure
+
+```json
+{
+  "outbounds": [
+    {
+      "type": "dns",
+      "tag": "dns-out"
+    }
+  ]
+}
+```
+
+!!! note ""
+
+    There are no outbound connections by the DNS outbound, all requests are handled internally.
+
+### Fields
+
+No fields.

docs/configuration/outbound/index.md

@@ -20,6 +20,7 @@
 | `socks`       | [Socks](./socks)             |
 | `http`        | [HTTP](./http)               |
 | `shadowsocks` | [Shadowsocks](./shadowsocks) |
+| `dns`         | [DNS](./dns)                 |
 | `selector`    | [Selector](./selector)       |
 | `urltest`     | [URLTest](./urltest)         |