update context menu docs to new content structure

docs/content/context-menu.md

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++++
+Categories = ["Widgets"]
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+
+A **context menu** allows users to take actions relevant to a [[widget]]. Users can open context menus by [[events#context menu|right clicking]] on a widget or [[events#long press]]ing down on a widget for 500 milliseconds, so context menus work on all platforms.
+
+You can add [[button]]s to the context menu of a widget:
+
+```Go
+tf := core.NewTextField(b)
+tf.AddContextMenu(func(m *core.Scene) {
+    core.NewButton(m).SetText("Build")
+    core.NewButton(m).SetText("Run")
+})
+```
+
+You can remove all of the context menu buttons of a widget:
+
+```Go
+tf := core.NewTextField(b)
+tf.ContextMenus = nil
+```
+
+## Scene context menu
+
+Note that there is still a context menu in the example above since all widgets inherit the [[scene]] context menu items, which consist of various important actions by default, such as the [[settings]] and [[inspector]]. You can remove these items if you want:
+
+```go
+myWidget.Scene.ContextMenus = nil
+```