Introduce docs for horizontal migration

[zuk3975]

Questions	Answers
Branch?	8.x
Description?	Moves all previous migration docs to dedicated "vertical-migration" directory and introduces additional "horizontal-migration" directory and the initial docs for it (probably starting by "why's" and how to migrate list - TBD later)
Fixed ticket?	started by https://github.com/PrestaShop/docs/pull/1401 but some concepts will change so I started it from scratch (copied some of info) and will replace that PR.