[Meta] [New docs] Custom docusaurus theme

[tkajtoch]

Summary

EUI's new documentation site is based on docusaurus, and it needs a custom theme that follows Elastic look and feel and uses EUI components.

Figma file: https://www.figma.com/design/g5Vk0k2DgeQPCPBoWmun2U/EUI%2B-docs?node-id=0-1&t=1MuoWGjtMjSkH3JF-0

Acceptance criteria

• The custom theme should be a separate package published as @elastic/eui-docusaurus-theme
• The theme should be implemented using docusaurus swizzle script and wrap original docusaurus components whenever possible to make it easier to upgrade docusaurus versions