Accessibility: Add a way to escape the jsx docblocks using the keyboard

[knitcodemonkey]

On the homepage of https://reactjs.org/, there are LIVE JSX EDITORs. For keyboard-only users, once you tab into them, how does one get out?

[michaelhiiva]

I would like to take on this issue if possible.

[alexkrolick]

@EarthSchlange great, it's yours! If you change your mind, just let us know and we'll remove the in-progress label.