extend divider to parent by setting props by default on the root container (BoxWithFallback) in the PageLayout component

[dwsosa]

Closes #4351

The PageLayout component is not opinionated about layout. By adjusting the props, it will fill its parent, and the divider will extend as expected:

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• [ ] Patch release
• [X] Minor release
• [ ] Major release; if selected, include a written rollout or migration plan
• [ ] None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation
• [ ] Added/updated previews (Storybook)
• [ ] Changes are SSR compatible
• [X] Tested in Chrome
• [ ] Tested in Firefox
• [ ] Tested in Safari
• [ ] Tested in Edge
• [ ] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 5cc8f5f6da3555b9263c1e8e0596a0a582d78efc

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

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Minor
@primer/styled-react	Major

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[lesliecdubs]

Re-requesting @jonrohan's review to see about getting workflows passing so we can merge this