docs(react guide): update guide to prevent memory leaks when components are disposed of (#4998)

When a component is disposed of, without calling ReactDOM.unmountComponentAtNode() on the data-reactroot in the destroyed component, the React.Component and videojs.Component (here EpisodeList and vjsEpisodeList) remain in memory.

The videojs.Component should listen for its dispose event and unmount its React root in the corresponding React.Component.

Author: EHummerston

docs/guides/react.md

@@ -105,6 +105,11 @@ class vjsEpisodeList extends vjsComponent {
     player.ready(() => {
       this.mount();
     });
+    
+    /* Remove React root when component is destroyed */
+    this.on("dispose", () => {
+      ReactDOM.unmountComponentAtNode(this.el())
+    });
   }
 
   /**