docs: add retention doc (#2892)

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site/content/docs/include/logging.en.md

@@ -3,6 +3,9 @@
 <a id="logging" href="#logging" class="field">`logging`</a> <span class="type">Map</span>  
 The logging section contains log configuration parameters for your container's [FireLens](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/using_firelens.html) log driver (see examples [here](../developing/sidecars.en.md#sidecar-patterns)).
 
+<span class="parent-field">logging.</span><a id="retention" href="#logging-retention" class="field">`retention`</a> <span class="type">Integer</span>  
+Optional. The number of days to retain the log events. See [this page](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-logs-loggroup.html#cfn-logs-loggroup-retentionindays) for all accepted values. If omitted, the default is 30.
+
 <span class="parent-field">logging.</span><a id="logging-image" href="#logging-image" class="field">`image`</a> <span class="type">Map</span>  
 Optional. The Fluent Bit image to use. Defaults to `amazon/aws-for-fluent-bit:latest`.