[exporter/file][chore][docs] Mention OTLP JSON File receiver in File …

…exporter docs (#35197)

Users have trouble finding the receiver that can read the data written
by File exporter back into the collector (e.g. see this [CNCF Slack
post](https://cloud-native.slack.com/archives/C01N6P7KR6W/p1726222186384309)).
Let's mention the OTLP JSON File receiver in the File exporter's docs.

exporter/fileexporter/README.md

@@ -13,6 +13,10 @@
 [contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 <!-- end autogenerated section -->
 
+Writes telemetry data to files on disk.
+
+Use the [OTLP JSON File receiver](../../receiver/otlpjsonfilereceiver/README.md) to read the data back into the collector (as long as the data was exported using OTLP JSON format).
+
 Exporter supports the following features：
 
 + Support for writing pipeline data to a file.