[New docs] Setup the custom docusaurus EUI theme as a separate package

[tkajtoch]

Summary

The upcoming new EUI documentation website needs a custom docusaurus theme that looks and feels like Elastic. We want to make the theme publicly available, so other Elastic teams can use it for their docusaurus-powered websites.

See https://github.com/facebook/docusaurus/tree/main/packages/docusaurus-theme-classic for docusaurus theme creation guidance.

Acceptance Criteria

• The custom theme should live in the packages/docusaurus-theme directory
• A separate package.json should be created in the theme's root directory in order to prepare it for future publishing
  • The package should be named @elastic/docusaurus-theme-eui
• The theme should contain all the theming changes made in feat/new-docs including EUI components and Emotion support