[docs] Improve server-rendering, replace render by hydrate

[Mystraht]

Replace render by hydrate to improve performance when JS client is loaded and avoid deprecation warning from React 16. (Render will be removed in React v17)
I also removed path import which was not used.

Source : https://stackoverflow.com/questions/46516395/whats-the-difference-between-hydrate-and-render-in-react-16

https://reactjs.org/docs/react-dom.html

Using ReactDOM.render() to hydrate a server-rendered container is deprecated and will be removed in React 17. Use hydrate() instead

[oliviertassinari]

@Mystraht Thank you