Add log and dns documentation

docs/configuration/dns/index.md

@@ -0,0 +1,40 @@
+### Structure
+
+```json
+{
+  "dns": {
+    "servers": [],
+    "rules": [],
+    "final": "",
+    "strategy": "prefer_ipv6",
+    "disable_cache": false,
+    "disable_expire": false
+  }
+}
+
+```
+
+### Fields
+
+| Key      | Format                         |
+|----------|--------------------------------|
+| `server` | List of [DNS Server](./server) |
+| `rules`  | List of [DNS Rule](./rule)     |
+
+#### final
+
+Default dns server tag, the first one will be used if it is empty.
+
+#### strategy
+
+Default domain strategy for resolving the domain names.
+
+One of `prefer_ipv4` `prefer_ipv6` `ipv4_only` `ipv6_only`.
+
+#### disable_cache
+
+Disable dns cache.
+
+#### disable_expire
+
+Disable dns cache expire.