PDF Download

## 🚀 Feature

The ability for users to download an offline version of the document.

This would be ideal as a PDF with title page and table of contents.

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

Yes

## Motivation

Interested in using this as a documentation solution for client producs but PDF for print is required by some.

## Pitch

A download button could be added to the top of the side nav which when clicked on could would download the file.