reactjs.org homepage examples are outdated

[slk333]

The examples are located here:
https://github.com/reactjs/reactjs.org/tree/master/content/home/examples
We can see that they received little to no change since their creation, more than 3 years ago.

There are two problems:

• The examples don't use hooks
• The examples use classes

More generally, reactjs.org introduction largely ignores hooks, while using classes throughout. As a result, hooks are largely ignored by newcomers.

Suggestion: the Clock example and the Markdown Editor examples should come in two alternative versions: one using Class, one using function components and useState() and useEffect() hooks.

[Zeko369]

I'm already working on this, will have a PR today

[slk333]

#2874 (comment)