docs: Add image to known issues section

Add image that makes it more clear how historical data looks
with clustered data points.

Signed-off-by: Simen S. Røstad <[email protected]>

Author: simensrostad

docs/common/known_issues.md

@@ -8,8 +8,10 @@ This page documents known issues and limitations across different components of
 
 **Issue:** The nRF Cloud historical data view displays the `receivedAt` timestamp (when the cloud received the message) instead of the sample timestamp from the message payload. This causes batched samples with different timestamps to appear clustered at the transmission time.
 
+<img src="../images/historical_data.png" alt="Historical data timestamp issue" width="800" />
+
 **Workaround:**
-- Use the live view (timestamps are correct there)
-- Use the [REST API](https://api.nrfcloud.com) to retrieve device messages (`GET /v1/messages?device_id={DEVICE_ID}`) and parse timestamps from the message payload
+- Use the live view, timestamps are correctly shown there.
+- Use the [REST API](https://api.nrfcloud.com) to retrieve device messages (`GET /v1/messages?device_id={DEVICE_ID}`) and parse timestamps from the message payload.
 - Use the [Message Routing Service](https://docs.nordicsemi.com/bundle/nrf-cloud/page/Devices/MessagesAndAlerts/MessageRoutingService/ReceivingMessages.html) to automatically forward device messages to your own infrastructure in real-time.
 - For production deployments, it's recommended to rely on programmatic data export (REST API or Message Routing Service) rather than the web portal for accurate timestamp handling and automated data processing.