feat(docusaurus-theme): improve document styling

[tkajtoch]

Summary

Resolves #7956

This PR updates docusaurus theme styles to improve readability and fix some text rendering issues:

• remove EuiText wrapper from MDXContent so that non-native elements like the <Demo> component don't get their styles overridden
• increase document content width to <=830px (results in paragraph widths of up to 802px)
• remove shadow from admonitions/callouts
• switch to a custom Heading component

The primary goal of these changes was to create a better content structure to increase readability on mobile and desktop.

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: bd3c9a0c11eac5e75d803522e5c7600ce3bf25df

History

• :green_heart: Build #2641 succeeded 7fc48851cd404743a676b19c81686e310ec24054

cc @tkajtoch