Move context menus out of experimental (#2134)

Now that the context menu code is on stable, move the sample out of
experimental.

Fixes https://github.com/flutter/samples/issues/2110

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

context_menus/README.md

@@ -0,0 +1,57 @@
+# Custom Context Menus
+
+This sample shows how to create and customize cross-platform context menus,
+such as the text selection toolbar on mobile or the right click menu on desktop.
+
+| ![Cascading example screenshot](https://raw.githubusercontent.com/flutter/samples/main/experimental/context_menus/screenshots/cascading.png) | ![Custom button example screenshot](https://raw.githubusercontent.com/flutter/samples/main/experimental/context_menus/screenshots/custom.png) | ![Email example screenshot](https://raw.githubusercontent.com/flutter/samples/main/experimental/context_menus/screenshots/email.png) | ![Widget example screenshot](https://raw.githubusercontent.com/flutter/samples/main/experimental/context_menus/screenshots/image.png) |
+| --- | --- | --- | --- |
+
+## Running the sample
+
+Just run `flutter run` in the same directory as this README file.
+
+## The examples
+
+### [Anywhere](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/anywhere_page.dart)
+Shows how to create a context menu in the parts of an app that don't related to
+text selection.  For example, a menu in a desktop app that shows when the
+background of the app is right clicked.
+
+### [Cascading menus](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/cascading_menu_page.dart)
+Shows how to create a context menu with cascading submenus using
+[SubmenuButton](https://master-api.flutter.dev/flutter/material/SubmenuButton-class.html).
+
+### [Custom buttons](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/custom_buttons_page.dart)
+Shows how to customize the default buttons in the existing context menus.
+
+### [Custom menu](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/custom_menu_page.dart)
+Shows how to use any custom widgets as the menu itself, including the option to
+keep the default buttons.
+
+### [Default values](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/default_values_page.dart)
+Demonstrates how the
+[contextMenuBuilder](https://master-api.flutter.dev/flutter/material/TextField/contextMenuBuilder.html)
+property works with various possible values.
+
+### [Email button](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/email_button_page.dart)
+Shows how to create an "email" button in the default context menu that shows
+only when an email address is selected.
+
+### [Field types](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/field_types_page.dart)
+Shows how context menus work in the various different field widgets:
+EditableText, TextField, and CupertinoTextField.
+
+### [Global selection](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/global_selection_page.dart)
+Shows how to create a custom context menu in non-editable selection with
+[SelectionArea](https://master-api.flutter.dev/flutter/material/SelectionArea-class.html).
+
+### [On a widget](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/image_page.dart)
+Shows how to make a widget show a context menu on right click or long press, in
+this case an Image widget.
+
+### [Modified action](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/modified_action_page.dart)
+Shows how to modify an existing button so that a custom action is performed when
+it is tapped.
+
+### [Reordered buttons](https://github.com/flutter/samples/blob/main/experimental/context_menus/lib/reordered_buttons_page.dart)
+Shows how to change the order of the default buttons.