Restructure plugin docs (#24381)

docs/sources/developers/plugins/legacy/panels.md

@@ -0,0 +1,28 @@
++++
+title = "Legacy panel plugins"
+keywords = ["grafana", "plugins", "panel", "documentation"]
+type = "docs"
+aliases = ["/docs/grafana/latest/plugins/developing/panels/"]
++++
+
+# Legacy panel plugins
+
+Panels are the main building blocks of dashboards.
+
+## Panel development
+
+### Scrolling
+The grafana dashboard framework controls the panel height.  To enable a scrollbar within the panel the PanelCtrl needs to set the scrollable static variable:
+
+```javascript
+export class MyPanelCtrl extends PanelCtrl {
+  static scrollable = true;
+  ...
+```
+
+In this case, make sure the template has a single `<div>...</div>` root.  The plugin loader will modify that element adding a scrollbar.
+
+### Examples
+
+- [clock-panel](https://github.com/grafana/clock-panel)
+- [singlestat-panel](https://github.com/grafana/grafana/tree/master/public/app/plugins/panel/singlestat)