[formrecognizer] Add migration guide (Azure#20960)

* add migration guide

* title fix

* add more examples

* update wording

* some review feedback and fixes

* more fixes

* additional doc fixes

* add full samples to guide

* improve content table, add more info and change subheader

* reorganize, add more info about model ids

* fix links

* fix relative links

* fix spelling errors, add Mari's feedback

* review feedback from Krista

* grammar

* add manage models section

* more feedback