[EUI+] Initial typography and doc content updates

[mgadewoll]

Summary

closes: #7795 relates to #7822

[!IMPORTANT] ~Parts of this PR (anything related to the ColorModeToggle) was added as a separate PR here but kept in this PR as well for testing purposes. That previous PR for ColorModeToggle should be reviewed and merged before this one.~ Update: The PR was merged and this PR rebased.

This PR updates the base global styles for the new EUI+ docs and additionally adds the first style updates for doc content components.

Changes:

• swizzles MDXContent (wrap) and updates it to use EuiText + custom EUI+ adjustments to ensure markdown content styling (text, lists etc)
• swizzles DocItem/Content (wrap) and adds custom styles for the document heading
• swizzles MDXComponents/A and MDXComponents/Code (wrap) and uses Eui components instead (EuiLink and EuiCode/EuiCodeBlock)
• swizzles MDXComponents/Heading and uses a wrapping EuiText to apply EUI heading styles
• adds a custom EUI+ CallOut component and uses it to wrap Docusaurus admonition components (Note, Info, Tip, Warning, Danger) and apply custom styles
• updates swizzled Footer component theme to ensure static dark theme (independent from global colorMode)

QA

• [ ] checkout this PR gh pr checkout 7848, build the theme in /packages/docusaurus-theme with yarn build and run the EUI+ docs page in packages/website with yarn start
  • [ ] review docs pages and styles and verify that they look as currently expected based on initial designs (link)