Adding grid usage explanation to app layout when l-nav is in is-collapsed state.

[camillet16]

Describe the bug

Lacking info on Vanilla docs

To Reproduce

Steps to reproduce the behavior:

1. Go to 'Vanilla docs app layout'
2. Missing content

Expected behavior

Show how grids work when l-nav is in an is-collapsed state and when l-aside is a panel not a slide over.

Screenshots

• When l-aside is a panel not a slide over

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• When l-nav is-collapsed - does the grid stretch across the screen or does it float left?

• OS: MacOS
• Browser: Chrome
• Version: Latest

Add any other context about the problem here.
Vanilla working group help :)