Add SSM API service

docs/configuration/service/ssm-api.md

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+---
+icon: material/new-box
+---
+
+!!! question "Since sing-box 1.12.0"
+
+# SSM API
+
+SSM API service is a RESTful API server for managing Shadowsocks servers.
+
+See https://github.com/Shadowsocks-NET/shadowsocks-specs/blob/main/2023-1-shadowsocks-server-management-api-v1.md
+
+### Structure
+
+```json
+{
+  "type": "ssm-api",
+  
+  ... // Listen Fields
+  
+  "servers": {},
+  "tls": {}
+}
+```
+
+### Listen Fields
+
+See [Listen Fields](/configuration/shared/listen/) for details.
+
+### Fields
+
+#### servers
+
+==Required==
+
+A mapping Object from HTTP endpoints to [Shadowsocks Inbound](/configuration/inbound/shadowsocks) tags.
+
+Selected Shadowsocks inbounds must be configured with [managed](/configuration/inbound/shadowsocks#managed) enabled.
+
+Example: 
+
+```json
+{
+  "servers": {
+    "/": "ss-in"
+  }
+}
+```
+
+#### tls
+
+TLS configuration, see [TLS](/configuration/shared/tls/#inbound).