[ch4109]

Closes https://github.com/vercel/next.js/issues/14500

Our current docs for `next/router` use examples from both `useRouter` and `import Router from 'next/router'`, with this PR I'm unifying the API into `useRouter` (`withRouter` is still going to be mentioned multiple times as it is the HOC alternative) and no longer mentioning that you can import a global `Router` object from `next/router`, not sure if I should mention it at least once but that didn't seem to be required.

I also did some structural changes to the docs for `next/router`, now every method starts with a description, then the implementation and explanation of the parameters of the method, and then the usage example, because every method uses the same `Usage` title the hash for them would be something like `#usage`, `#usage-1`, `#usage-2`, e.t.c, so I'm not very happy with this but it looks good.

Feedback wanted 🙏