docs(side-panel): add SiSidePanelService API docs

Author: ljanner

docs/components/layout-navigation/side-panel.md

@@ -176,4 +176,6 @@ actions, or status indicators.
 
 <si-docs-api component="SiSidePanelContentComponent"></si-docs-api>
 
+<si-docs-api injectable="SiSidePanelService"></si-docs-api>
+
 <si-docs-types></si-docs-types>