Update the decorators documentation (Stage 3)

Finally the decorators have been moved to stage 3!

https://github.com/tc39/proposal-decorators/pull/458

Babel plugin configuration has changed:
https://babeljs.io/docs/en/babel-plugin-proposal-decorators#options

Current Lit documentation:
https://lit.dev/docs/components/decorators/

I think we should update it.