Docs: loki.md: Add example of Loki data source config (#29976)

* loki.md: Add example of Loki data source config

It was little difficult to search an example of setting datasourceUid for Jaeger data source. 

I found the answer from https://github.com/grafana/grafana/issues/26087 and thought it would be good to document it.

* Update docs/sources/datasources/loki.md

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Update docs/sources/datasources/loki.md

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

docs/sources/datasources/loki.md

@@ -231,3 +231,15 @@ datasources:
           name: TraceID
           url: "http://localhost:16686/trace/$${__value.raw}"
 ```
+
+Here's an example of a Jaeger data source corresponding to the above example. Note that the Jaeger `uid` value does match the Loki `datasourceUid` value.
+
+```
+datasources:
+    - name: Jaeger
+      type: jaeger
+      url: http://jaeger-tracing-query:16686/
+      access: proxy
+      # UID should match the datasourceUid in dervidedFields.
+      uid: my_jaeger_uid
+```