Add rector / upgrade repo into migration guides

[dereuromark]

Rector should be mentioned in the migration guides - or have a quicklink there

Only the upgrade guide mentions it https://book.cakephp.org/5/en/appendices/5-0-upgrade-guide.html But

The upgrade tool only works on applications running on latest CakePHP 4.x. You cannot run the upgrade tool after updating to CakePHP 5.0.

not sure thats a valid case as well, or at least not for the other minors then. So either way we gotto refactor this info a bit.