Site Editor: remove outer content margins to better match front-end

[creativecoder]

Description

This change demonstrate removing the outer content margins from the site editor, to better match how the content looks on the front-end of the site (addresses #29357)

How has this been tested?

• Using TT1 Blocks theme and editing a template
• Manually checking the margins in the site editor are removed
• Manually checking that changes do not bleed into the post/page editor

Screenshots

Header	Footer

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

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[creativecoder]

The site editor no longer shows the extra margins--closing this.