[New docs] Fix `EuiProvider`'s `globalStyles` breaking docusaurus styles about eui HOT 4 OPEN

This PR updates the EUI EuiGlobalStyles to support using partial global styles. There will be an additional small PR to add the update to the EUI+ theme but any further EUI+ specific changes will be done in the design update tasks, e.g. for updating the typography in #7795

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I'm generally not a huge fan of adding functionality to production components for the sake of non-production use-cases. Can I ask why we can't just disable EUI's global styles completely via <EuiProvider globalStyles={false} /> and then just manually import or copy/paste the styles we do want to Docusaurus? That would personally by my preference (putting logic/customization in the lower stakes repo).

from eui.

The idea was to be able to reuse some of the things where needed by making this more flexible. But I can agree that:

• it makes sense to setup things project independent (though the idea in EUI+ is to reuse as much as possible and the theme is opinionated)
• the EUI+ usage would be a one-off so far and we don't fully know yet what exactly will be used and how down the line as we only just start implementing

I think we could work also with globalStyles={false} for now and see later in the process how useful the granular update of EuiGlobalStyles will be and decide then if we want to move forward with this? cc @tkajtoch

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The second bullet point is definitely my concern! I think I just also don't want to spend too much time reviewing/working on future-proof logic on source code when we don't have clear production usages in mind. Hope that makes sense!

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