📝 document console sink types

docs/api/console-sink-type.md

@@ -0,0 +1,72 @@
+---
+name: console-sink-type
+group: api
+category: sink
+update-time: 20260613
+description: Public plain console sink type used for default text console output.
+key-word:
+    - sink
+    - console
+    - type
+    - public
+---
+
+## Console-sink-type
+
+`ConsoleSink` is the public plain console sink type used for default text console output. It is the minimal built-in synchronous sink and serves as the concrete sink type returned by `console_sink()`.
+
+### Interface
+
+```moonbit
+pub struct ConsoleSink {
+  _dummy : Unit
+}
+```
+
+#### output
+
+- `ConsoleSink` - Public synchronous sink type that writes records to the console with default text formatting.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a public root struct, not a type alias.
+- The current visible field is `_dummy : Unit`, which exists only to carry the concrete sink type.
+- `console_sink()` constructs this type as the simplest built-in console output path.
+- `Logger[ConsoleSink]`, `LibraryLogger[ConsoleSink]`, and default logger helpers all use this concrete sink type when no richer formatting or routing layer is introduced.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Typed Plain Console Sink Value
+
+When code should keep the concrete sink type visible:
+```moonbit
+let sink : ConsoleSink = console_sink()
+```
+
+In this example, the sink value stays explicit and can be passed into typed logger construction.
+
+#### When Need A Minimal Root Logger Type
+
+When logging should start from the simplest built-in console sink:
+```moonbit
+let logger : Logger[ConsoleSink] = Logger::new(console_sink(), target="app")
+```
+
+In this example, the concrete console sink type remains part of the logger type.
+
+### Error Case
+
+e.g.:
+- `ConsoleSink` itself does not have a runtime failure mode.
+
+- Console output still depends on the current runtime environment even though the sink type is always constructible.
+
+### Notes
+
+1. Use `console_sink()` when you need a value of this type.
+
+2. Use `JsonConsoleSink` or `FormattedConsoleSink` when output shape should be more explicit than the default text path.