add data logging doc

docs/data-logging.md

@@ -0,0 +1,10 @@
+# Data Logging
+
+When Data Logging is enabled (See parameter [`DataLogActive`](../Parameters/#parameter-datalogactive)),
+the results of every round gets written to the SD-Card.
+
+The data files are stored in `/log/data` on the SD-Card.
+
+## Data Format
+The data is stored as CSV with the following columns:
+`time`, `name-of-number`, `raw-value`, `return-value`, `pre-value`, `change-rate`, `change-absolute`, `error-text`, `cnn-digital`, `cnn-analog`

docs/nav.yml

@@ -29,6 +29,7 @@ nav:
     - Parameters.md
     - Integration-Home-Assistant.md
     - External-LED.md
+    - data-logging.md
     - rolling-installation.md
     - Watermeter-specific-analog---digital-transition.md
     - collect-new-images.md