All markdown docs on one page

## 🚀 Feature

Allow all your documentation to be on one page instead of separate links

## Have you read the [Contributing Guidelines on issues](https://github.com/facebook/Docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

Yes.

## Motivation

via @jordwalke

## Pitch

Some people like to just see everything on one screen without having to click around (e.g., `prev`, `next`).