Add keyboard method to the user-event page

[luizbaldi]

The problem
I had to use the user-event .keyboard() API method to simulate a keyboard event, but I found that it's not part of the official doc, even though it was released some time ago.

The solution
It would be nice to have the user-event keyboard API method being part of the official documentation website.

Additional context
I'm more than happy to fork the repo and add it to the page, but I just wanted to double-check first if this actually makes sense before jumping in.