[docs] `useRouter` cannot be used with classes (#8649)
[docs] `useRouter` cannot be used with classes (#8649)

The `useRouter` function is a hook and cannot be used with Classes.

A few React beginners have brought this up, so we document the alternatives to hooks.
zeit/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,flybayer/next.js,BlancheXu/test,zeit/next.js,azukaru/next.js,JeromeFitz/next.js,BlancheXu/test,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,zeit/next.js,flybayer/next.js,azukaru/next.js,BlancheXu/test