Add two sidecar layouts to layout component

[wibjorn]

Task: task-925514
Task: task-925513

Link: preview-586
Page specific link: https://design.learn.microsoft.com/pulls/586/components/layout.html

This pull request adds a third possible layout to the .layout component. See documentation page for details. Also fixes a mobile navigation issue introduced late in the review process for #582

Testing

1. Visit documentation page.
2. Select "Show container labels" button
3. Select "Sidecar left" and "Sidecar right" from the layout buttons.
4. Read through code.

Contributor checklist

• Did you update the description of this pull request with a review link and test steps?
• Did you submit a changeset? Changesets are required for all code changes.
• Does your pull request implement complex UI logic (js/ts)? Consider adding an integration test to test your user flow.
• Does your pull request add a new page to the documentation site? Add your new page for automated accessibility testing in /integration/tests/accessibility.

[changeset-bot]

🦋 Changeset detected

Latest commit: 4c57a4c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@microsoft/atlas-site	Minor
@microsoft/atlas-css	Minor

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