📝 Add sync config export API docs

docs/api/sink-config-to-json.md

@@ -0,0 +1,97 @@
+---
+name: sink-config-to-json
+group: api
+category: config
+update-time: 20260512
+description: Convert SinkConfig into a JSON value for export and nested logger config serialization.
+key-word:
+    - sink
+    - config
+    - json
+    - public
+---
+
+## Sink-config-to-json
+
+Convert `SinkConfig` into a `JsonValue`. This helper is used directly for sink export and indirectly when exporting `LoggerConfig`.
+
+### Interface
+
+```moonbit
+pub fn sink_config_to_json(config : SinkConfig) -> @json_parser.JsonValue {}
+```
+
+#### input
+
+- `config : SinkConfig` - Sink config to export.
+
+#### output
+
+- `JsonValue` - JSON representation of the sink configuration.
+
+---
+
+e.g.:
+```moonbit
+pub fn sink_config_to_json(config : SinkConfig) -> @json_parser.JsonValue {}
+```
+
+#### input
+
+- `config : SinkConfig` - Sink definition.
+
+#### output
+
+- `JsonValue` - Structured JSON sink value.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The output includes `kind`, `path`, `append`, `auto_flush`, and `text_formatter`.
+- `rotation` is only included when present.
+- The exported shape is schema-oriented and shared by larger config export helpers.
+- This helper is especially useful when building larger config payloads manually.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Sink-only Export
+
+When only the sink portion should be emitted:
+```moonbit
+let json = sink_config_to_json(SinkConfig::new(kind=SinkKind::TextConsole))
+```
+
+In this example, sink config can be exported independently of the full logger config.
+
+#### When Compose A Larger JSON Payload
+
+When sink config should be embedded elsewhere:
+```moonbit
+let sink_json = sink_config_to_json(config.sink)
+```
+
+In this example, sink config becomes a structured JSON component.
+
+### Error Case
+
+e.g.:
+- If `rotation` is absent, it is simply omitted from the exported sink config.
+
+- If file-related fields are unused by the sink kind, they are still exported according to the stable config shape.
+
+### Notes
+
+Notes are here.
+
+1. This API is sink-config export only, not runtime sink inspection.
+
+2. Use `stringify_sink_config(...)` for direct string output.
+
+3. This helper is reused by `logger_config_to_json(...)`.
+
+4. It is useful for tooling that needs nested config JSON values.