🤖 docs: Reference regenerator-runtime instead of @babel/polyfill.

Replace all references to @babel/polyfill by references to regenerator-runtime/runtime.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/make-github-pseudonymous-again/rejuvenate/blob/9b50c2a1d7e9ab7b4bc9c9ff88892e98c5f62244/src/transforms/docs:replace-scoped-babel-polyfill-with-regenerator-runtime.js
Please contact the author of the transform if you believe there was an error.

Author: a-flying-potato

doc/manual/usage.md

@@ -1,13 +1,13 @@
 # Usage
 
 > :warning: The code needs a ES2015+ polyfill to run (`regeneratorRuntime`),
-> for instance [@babel/polyfill](https://babeljs.io/docs/usage/polyfill).
+> for instance [regenerator-runtime/runtime](https://babeljs.io/docs/usage/polyfill).
 
 First, require the polyfill at the entry point of your application
 ```js
-require( '@babel/polyfill' ) ;
+require( 'regenerator-runtime/runtime' ) ;
 // or
-import '@babel/polyfill' ;
+import 'regenerator-runtime/runtime' ;
 ```
 
 Then, import the library where needed