Add documentation for wasm, dual-compile, and new `FlutterLoader.load` APIs

[eyebrowsoffire]

For the dart2wasm+skwasm effort, there are a number of thing's I will need to write some documentation on and publish including:

• The --wasm flag, and how it produces a dual-compiled application bundle
• Templating of the .html file to inject build configuration in for dual compiled builds
• Use of the FlutterLoader.load API which is the new canonical way to load your app
• How to serve your files with COOP and COEP headers in order to get crossOriginIsolated so users can get multi-threaded rendering via skwasm

[eyebrowsoffire]

Some stuff is in a draft here: https://github.com/flutter/website/pull/10256

[felipecastrosales]

hey, stable version was released yesterday and this looks like confusing. any expectations about it?

[eyebrowsoffire]

We added documentation here: https://docs.flutter.dev/platform-integration/web/wasm

And here: https://docs.flutter.dev/platform-integration/web/bootstrapping

Closing this as completed.