Add netns and unshare support

docs/configuration/network-namespace/unshare.md

@@ -0,0 +1,36 @@
+---
+icon: material/new-box
+---
+
+!!! question "Since sing-box 1.14.0"
+
+# Unshare
+
+Create a new network namespace, without root privilege.
+
+!!! info ""
+
+    Rootless operation requires the kernel to allow unprivileged user namespace creation.
+
+### Structure
+
+```json
+{
+  "network_namespaces": [
+    {
+      "type": "unshare",
+      "tag": "",
+      "pid_file": ""
+    }
+  ]
+}
+```
+
+### Fields
+
+#### pid_file
+
+If set, the PID of the process holding the namespace open is written to this path.
+
+The namespace can be entered with `nsenter -t <pid> -n` when sing-box is run as root,
+or `nsenter -t <pid> -U --preserve-credentials -n` otherwise.