Description

This PR introduces the migration of the alpha and legacy checkout pages from the pages directory, to the app router.

Issues

Fixes # Refs #13673

[x] 1 PR, 1 purpose: my Pull Request applies to a single purpose
[x] I have commented my code, particularly in hard-to-understand areas
[ ] I have updated the docs to reflect my changes if applicable
[ ] I have added tests (and stories for frontend components) that prove my fix is effective or that my feature works
[x] I have performed a self-review of my own code
[ ] If my code involves visual changes, I am adding applicable screenshots to this thread