RFC: Keyboard Shortcuts

Initially proposed by @yangshun here: https://github.com/facebook/docusaurus/issues/2267

## 💥 Proposal: Keyboard Shortcuts

**Difficulty**: Easy-Medium

How it works - plugins can register keyboard shortcuts which Docusaurus will attach as a global event listener and show a list of all supported shortcuts on the website when the user types ?, like Reddit:

Of course it'll be up to the plugins to define their own logic for responding to events. Docusaurus can provide the keyboard event listening mechanism and call plugin code when a specific key combination occurs.