docs: add guide to separate configuration files

docs/en/configuration/separate-config.md

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+# Separate Configuration Files
+
+Sometimes you want to break your configuration file into several files. It may be useful in the following cases:
+
+1. You want to switch nodes via modify the config file using tools like `sed`.
+2. You copy other's configuration file and you want to overwrite some parts of it.
+
+## Example
+
+Directory Structure:
+
+```sh
+# tree /etc/dae
+/etc/dae
+├── config.d
+│  ├── dns.dae
+│  ├── node.dae
+│  └── route.dae
+└── config.dae
+```
+
+Config files:
+
+```jsonc
+# config.dae
+
+# load all dae files placed in ./config.d/
+include {
+    config.d/*.dae
+}
+global {
+    tproxy_port: 12345
+
+    log_level: warn
+
+    tcp_check_url: 'http://cp.cloudflare.com'
+    udp_check_dns: 'dns.google.com:53'
+    check_interval: 600s
+    check_tolerance: 50ms
+
+    #lan_interface: eth0
+    wan_interface: eth0
+    allow_insecure: false
+
+    dial_mode: domain
+    disable_waiting_network: false
+    auto_config_kernel_parameter: true
+    sniffing_timeout: 100ms
+}
+```
+
+```jsonc
+# dns.dae
+dns {
+    upstream {
+        alidns: 'udp://dns.alidns.com:53'
+        googledns: 'tcp+udp://dns.google.com:53'
+    }
+
+    routing {
+        request {
+            qname(geosite:category-ads) -> reject
+            qname(geosite:category-ads-all) -> reject
+            fallback: alidns
+        }
+        response {
+            upstream(googledns) -> accept
+            !qname(geosite:cn) && ip(geoip:private) -> googledns
+            fallback: accept
+        }
+    }
+}
+```
+
+```jsonc
+# node.dae
+node {
+    node1: 'xxx'
+    node2: 'xxx'
+}
+
+subscription {
+    my_sub: 'https://www.example.com/subscription/link'
+}
+
+group {
+    my_group {
+        filter: subtag(my_sub) && !name(keyword: 'ExpireAt:')
+        policy: min_moving_avg
+    }
+
+    local_group {
+        filter: name(node1, node2)
+        policy: fixed(0)
+    }
+}
+```
+
+```jsonc
+# route.dae
+routing {
+    pname(NetworkManager) -> direct
+    dip(224.0.0.0/3, 'ff00::/8') -> direct
+    dip(geoip:private) -> direct
+
+    dip(1.14.5.14) -> direct
+
+    domain(geosite:openai) -> local_group
+    dip(geoip:cn) -> direct
+    domain(geosite:cn) -> direct
+    domain(geosite:category-scholar-cn) -> direct
+    domain(geosite:geolocation-cn) -> direct
+
+
+    fallback: my_group
+}
+```
+
+Then run `dae` via:
+
+```sh
+dae run -c /etc/dae/config.dae
+```