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

**Describe the bug**

Lacking info on Vanilla docs

**To Reproduce**

Steps to reproduce the behavior:

1. Go to ['Vanilla docs app layout'](https://vanillaframework.io/docs/layouts/application)
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 :)

Open the linked Vanilla docs application-layout page and review its current grid guidance. Confirm the intended behavior for a collapsed l-nav and for l-aside as a panel rather than a slide-over, then add documentation showing both cases; done when users can determine how the grid is positioned in each state.

Written by the indexing model from the issue text.