Cover functional components and hooks in React guide

[delawski]

The current React guide should be updated to cover the latest best practices, i.e. using simple functional components along with both built-in and custom hooks.

Hooks and functional components are becoming an important part of React apps development, slowly but surely superseding the "traditional" class components and higher-order components.

Our docs should reflect this shift, promoting the most up-to-date approach to React development.