Documentation: Add pre-migration task to menu links migration docs.

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Description

Updating Readme to help anyone following the migration guide.

How to test

Types of changes

Arizona Quickstart (install profile, custom modules, custom theme)

• Patch release changes
  • Bug fix
  • Accessibility, performance, or security improvement
  • Critical institutional link or brand change
  • Adding experimental module
  • Update experimental module
• Minor release changes
  • New feature
  • Breaking or visual change to existing behavior
  • Upgrade experimental module to stable
  • Enable existing module by default or database update
  • Non-critical brand change
  • New internal API or API improvement with backwards compatibility
  • Risky or disruptive cleanup to comply with coding standards
  • High-risk or disruptive change (requires upgrade path, risks regression, etc.)
• Other or unknown
  • Other or unknown

Drupal core

• Patch release changes
  • Security update
  • Patch level release (non-security bug-fix release)
  • Patch removal that's no longer necessary
• Minor release changes
  • Major or minor level update
• Other or unknown
  • Other or unknown

Drupal contrib projects

• Patch release changes
  • Security update
  • Patch or minor level update
  • Add new module
  • Patch removal that's no longer necessary
• Minor release changes
  • Major level update
• Other or unknown
  • Other or unknown

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.