[New docs] Fix document styling issues

[tkajtoch]

Summary

The new documentation website has a few minor styling issues affecting the look of the content and rendered component previews:

1. The MDXContent default styles affect and override some of our component styles (e.g. ol and heading elements margins)
2. The Tabs bottom margin should not be set
3. The overridden body color for the light color scheme breaks some of the EUI components that use it as its subdued color variant
4. The content section width should be slightly increased to better fit our docs content