dns: Add mDNS server

docs/configuration/dns/server/mdns.md

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+---
+icon: material/new-box
+---
+
+!!! question "Since sing-box 1.14.0"
+
+# mDNS
+
+### Structure
+
+```json
+{
+  "dns": {
+    "servers": [
+      {
+        "type": "mdns",
+        "tag": "",
+
+        "interface": [],
+
+        // Dial Fields
+      }
+    ]
+  }
+}
+```
+
+!!! info ""
+
+    You usually do not need an explicit `mdns` server in addition to a [Local](./local/) server: the local server already routes queries for `*.local.` and IPv4/IPv6 link-local reverse zones via mDNS on non-Apple platforms and via the system resolver on Apple platforms. Add an explicit `mdns` server only when you want to reference it from [`preferred_by`](../rule/#preferred_by) or use it standalone.
+
+### Fields
+
+#### interface
+
+List of network interface names to send mDNS queries on.
+
+When empty, all interfaces that are up, multicast-capable, and non-loopback are used.
+
+### Dial Fields
+
+See [Dial Fields](/configuration/shared/dial/) for details.