Restored Skip to Main Content functionality (CRASM-630)

[hawkishpolicy]

• Restored "Skip to Main Content" functionality.

🗣 Description

• Added the "main-content" id to the Box div inside of Main.
• Set the tabIndex to -1 for the Box div inside of Main.

💭 Motivation and context

• The refactor of Layout following the creation of the new Side Drawer removed the functionality of the "Skip to Main Content" component.
• The restoration of the "main-content" id and -1 tabIndex restores this functionality.
• Closes CRASM-630
• Closes #589

🧪 Testing

📷 Screenshots (if appropriate)

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [ ] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [ ] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.

✅ Pre-merge checklist

• [ ] Revert dependencies to default branches.
• [ ] Finalize version.

✅ Post-merge checklist

• [ ] Create a release.