docs: added versions_to_keep to config docs, #9671

docs/sources/installation/configuration.md

@@ -624,6 +624,12 @@ Analytics ID here. By default this feature is disabled.
 
 <hr />
 
+## [dashboards]
+
+### versions_to_keep (introduced in v5.0)
+
+Number dashboard versions to keep (per dashboard). Default: 20, Minimum: 1.
+
 ## [dashboards.json]
 
 If you have a system that automatically builds dashboards as json files you can enable this feature to have the