[New docs] Move docusaurus website to `packages` directory

[tkajtoch]

Summary

As part of the EUI+ initiative, we want to have a new documentation website that's completely separate from EUI source code. This way the new website uses @elastic/eui as a dependency and has access to the same exports as regular customers instead of importing directly from the src directory as the current documentation website living in src-docs does.

Acceptance Criteria

• docusaurus-based EUI+ website should live in packages/website
• the deployment script should be adjusted to run yarn build in the updated directory