document add-to-app for web (#11056)

_Description of what this PR is changing or adding, and why:_

The web supports add-to-app via multi-view. This PR updates the
add-to-app docs to include web. I also attempt to improve some existing
text.

src/content/add-to-app/index.md

@@ -6,26 +6,37 @@ description: Adding Flutter as a library to an existing Android or iOS app.
 
 ## Add-to-app
 
-It's sometimes not practical to rewrite your entire application in
-Flutter all at once. For those situations,
-Flutter can be integrated into your existing
-application piecemeal, as a library or module.
-That module can then be imported into your Android or iOS
-(currently supported platforms) app to render a part of your
-app's UI in Flutter. Or, just to run shared Dart logic.
-
-In a few steps, you can bring the productivity and the expressiveness of
-Flutter into your own app.
-
-The `add-to-app` feature supports integrating multiple instances of any screen size.
+If you are writing a new application from scratch, it is easy to [get started][]
+using Flutter. But what if you already have an app that's not written in
+Flutter, and it's not practical to start from scratch?
+
+For those situations, Flutter can be integrated into your existing application
+piecemeal, as a module. This feature is known as "add-to-app". The module can be
+imported into your existing app to render part of your app using Flutter, while
+the rest can be rendered using existing technology. This method can also be used
+to run shared non-UI logic by taking advantage of Dart's portability and
+interopability with other languages.
+
+Add-to-app is currently supported on Android, iOS, and web.
+
+Flutter supports two flavors of add-to-app:
+
+- **Multi-engine**: supported on Android and iOS, allows running one or more
+ instances of Flutter, each rendering a widget embedded into the host
+ application. Each instance is a separate Dart program, running in isolation
+ from other programs. Having multiple Flutter instances allows each instance to
+ maintain independent application and UI state while using minimal memory
+ resources. See more in the [multiple Flutters][] page.
+- **Multi-view**: supported on the web, allows creating multiple
+ [FlutterView][]s, each rendering a widget embedded into the host application.
+ In this mode there's only one Dart program and all views and widgets can share
+ objects.
+
+Add-to-app supports integrating multiple instances of any screen size.
 This can help scenarios such as a hybrid navigation stack with mixed
 native and Flutter screens, or a page with multiple partial-screen Flutter
 views.
 
-Having multiple Flutter instances allows each instance to maintain
-independent application and UI state while using minimal
-memory resources. See more in the [multiple Flutters][] page.
-
 ## Supported features
 
 ### Add to Android applications
@@ -72,6 +83,32 @@ See our [add-to-app GitHub Samples repository][]
 for sample projects in Android and iOS that import
 a Flutter module for UI.
 
+### Add to web applications
+
+Flutter can be added to any existing HTML DOM-based web app written in any
+client-side Dart web framework ([jaspr][], [ngdart][], [over_react][], etc),
+any client-side JS framework ([React][], [Angular][], [Vue.js][], etc),
+any server-side rendered framework ([Django][], [Ruby on Rails][],
+[Apache Struts][], etc), or even no framework (affectionately known as
+"[VanillaJS][]"). The minimum requirement is only that your existing application
+and its framework support importing JavaScript libraries, and creating HTML
+elements for Flutter to render into.
+
+To add Flutter to an existing app, build it normally, then follow the
+[embedding instructions][] for putting Flutter views onto the page.
+
+[jaspr]: https://pub.dev/packages/jaspr
+[ngdart]: https://pub.dev/packages/ngdart
+[over_react]: https://pub.dev/packages/over_react
+[React]: https://react.dev/
+[Angular]: https://angular.dev/
+[Vue.js]: https://vuejs.org/
+[Django]: https://www.djangoproject.com/
+[Ruby on Rails]: https://rubyonrails.org/
+[Apache Struts]: https://struts.apache.org/
+[VanillaJS]: http://vanilla-js.com/
+[embedding instructions]: {{site.docs}}/platform-integration/web/embedding-flutter-web#embedded-mode
+
 ## Get started
 
 To get started, see our project integration guide for
@@ -92,6 +129,13 @@ Android and iOS:
   </header>
  </div>
  </a>
+ <a class="card" href="/platform-integration/web/embedding-flutter-web#embedded-mode">
+ <div class="card-body">
+  <header class="card-title text-center">
+  Web
+  </header>
+ </div>
+ </a>
 </div>
 
 ## API usage
@@ -114,17 +158,35 @@ see our API usage guides at the following links:
   </header>
  </div>
  </a>
+ <a class="card" href="/platform-integration/web/embedding-flutter-web#manage-flutter-views-from-js">
+ <div class="card-body">
+  <header class="card-title text-center">
+  Web
+  </header>
+ </div>
+ </a>
 </div>
 
 ## Limitations
 
+Mobile limitations:
+
 * Packing multiple Flutter libraries into an
  application isn't supported.
 * Plugins that don't support `FlutterPlugin` might have unexpected
  behaviors if they make assumptions that are untenable in add-to-app
  (such as assuming that a Flutter `Activity` is always present).
 * On Android, the Flutter module only supports AndroidX applications.
 
+Web limitations:
+
+* Multi-engine mode is not supported.
+* There's no way to completely "shutdown" the Flutter engine. The app can remove
+ all the [FlutterView][] objects and make sure all data is garbage collected
+ using normal Dart concepts. However, the engine will remain warmed up, even if
+ it's not rendering anything.
+
+[get started]: /get-started/codelab
 [add-to-app GitHub Samples repository]: {{site.repo.samples}}/tree/main/add_to_app
 [Android Archive (AAR)]: {{site.android-dev}}/studio/projects/android-library
 [Flutter plugins]: {{site.pub}}/flutter
@@ -138,3 +200,4 @@ see our API usage guides at the following links:
 [iOS Framework]: {{site.apple-dev}}/library/archive/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WhatAreFrameworks.html
 [maintained by the Flutter team]: {{site.repo.packages}}/tree/main/packages
 [multiple Flutters]: /add-to-app/multiple-flutters
+[FlutterView]: https://api.flutter.dev/flutter/dart-ui/FlutterView-class.html