[docs] Document Observe eval JSON paths

src/pages/docs/observe/guides/setup-evals.mdx

@@ -38,6 +38,20 @@ The name autofills. For a built-in eval the instructions and output type are pre
 - **Error localization** (optional): tick it to pinpoint which part of the input caused a failure
 - **Variable mapping** (required): map each of the eval's inputs to your data. The choices are the attributes your project actually carries at the level you picked: span attribute keys for **Spans**, trace fields (`input`, `output`, `metadata`, and more) plus paths into each span for **Traces**, and session fields plus paths into their traces and spans for **Sessions**. Here, map the eval's `output` to the trace's `output`
 
+### Map nested JSON attributes
+
+Use dot notation to map a nested object field. For arrays, use the numeric item position as a path segment, such as `.0` or `.1`; bracket notation such as `[0]` is not supported.
+
+A mapping can also start from a top-level span attribute whose value is a JSON object or array encoded as a string. The resolver parses that attribute, then continues walking the path.
+
+```json
+{
+  "attr_1": "{\"x\": {\"y\": [{\"z\": \"Your refund has been approved and will appear in your account within five business days.\"}]}}"
+}
+```
+
+Map the eval variable to `attr_1.x.y.0.z` to pass `Your refund has been approved and will appear in your account within five business days.` to the evaluator.
+
 Click **Add Evaluation** to attach it to the task.
 
 <img src="/images/docs/observe/evals-configure.png" alt="Toxicity eval configuration showing the read-only instructions, Pass/fail output, an error-localization checkbox, and a variable-mapping panel" style={{ borderRadius: '5px' }} />