GAUD-10314 - Simplify the z-index in POC

[svanherk]

Expanding on #7235, this shows how leaning into a stacking context future allows us to reduce the d2l-page z-index to a simple:

Header (2)
Side Nav (0) | Divider (1) | Main (0) | Divider (1) | Supporting (0)
Footer (1)

[github-actions]

Thanks for the PR! 🎉

We've deployed an automatic preview for this PR - you can see your changes here:

URL	https://live.d2l.dev/prs/BrightspaceUI/core/pr-7236/

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