Add blank Docusaurus site

[andyw8]

Motivation

This PR adds an empty documentation site powered by Docusaurus, so that we have a starting point for future PRs, based on what I started at Hackdays.

I've also added it as a new workspace in ruby-lsp.code-workspace.

We already have a docs/ directory so I named this docusaurus/ to distinguish it, but /docs would be nicer, so we could rename the current one.

Implementation

Ran npx create-docusaurus@latest and chose the classic theme.

I've chosen to use Yarn since we already use that on the extension.

Automated Tests

Nothing yet.

Manual Tests

Run yarn start and verify the site loads for you.