Add outbound documentation

docs/configuration/outbound/direct.md

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+`direct` outbound send requests directly.
+
+### Structure
+
+```json
+{
+  "outbounds": [
+    {
+      "type": "direct",
+      "tag": "direct-out",
+      
+      "override_address": "1.0.0.1",
+      "override_port": 53,
+
+      "detour": "upstream-out",
+      "bind_interface": "en0",
+      "routing_mark": 1234,
+      "reuse_addr": false,
+      "connect_timeout": "5s",
+      "tcp_fast_open": false,
+      "domain_strategy": "prefer_ipv6",
+      "fallback_delay": "300ms"
+    }
+  ]
+}
+```
+
+### Direct Fields
+
+#### override_address
+
+Override the connection destination address.
+
+#### override_port
+
+Override the connection destination port.
+
+### Dial Fields
+
+#### detour
+
+The tag of the upstream outbound.
+
+Other dial fields will be ignored when enabled.
+
+#### bind_interface
+
+The network interface to bind to.
+
+#### routing_mark
+
+The iptables routing mark.
+
+Only available in linux.
+
+#### reuse_addr
+
+Reuse listener address.
+
+#### connect_timeout
+
+Connect timeout, in golang's Duration format.
+
+A duration string is a possibly signed sequence of
+decimal numbers, each with optional fraction and a unit suffix,
+such as "300ms", "-1.5h" or "2h45m".
+Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
+
+#### domain_strategy
+
+One of `prefer_ipv4` `prefer_ipv6` `ipv4_only` `ipv6_only`.
+
+If set, the requested domain name will be resolved to IP before routing.
+
+`dns.strategy` will be used if empty.
+
+#### fallback_delay
+
+The length of time to wait before spawning a RFC 6555 Fast Fallback connection.
+That is, is the amount of time to wait for IPv6 to succeed before assuming
+that IPv6 is misconfigured and falling back to IPv4 if `prefer_ipv4` is set.
+If zero, a default delay of 300ms is used.
+
+Only take effect when `domain_strategy` is `prefer_ipv4` or `prefer_ipv6`.