Scrolling up from bottom doesn't update hash

- Right now, if you scroll down and go back to the top, #quick-start is kept in the hash
- If you go to https://docup.now.sh/#embeding for example and start to scroll up, technically you're already reading "Message Blocks", but the hash/active link doesn't change until you reach the header.

It's probably related to the headings being used as targets https://github.com/egoist/docup/blob/master/src/components/App.tsx#L13

Wrapping the sections with an element like a div could solve these issues.

Nice project btw 🎉