[Docs] Migration Guide

[JojoFR1]

Since the v2.x.y will bring huge changes, a migration guide is required to help users migrate from the v1.x.y to the v2.x.y

[JojoFR1]

The page is up at https://liplum.github.io/MultiCrafterLib/migration/.

It will be updated as new breaking changes will be introduced.