[WIP] v5: Prep migration docs page (#28472)
[WIP] v5: Prep migration docs page (#28472)

* Migration docs page for v5

* Add JS section, tweak some copy

* Update migration.md

peterblazejewicz/bootstrap,zalog/bootstrap,kvlsrg/bootstrap,stanwmusic/bootstrap,GerHobbelt/bootstrap,GerHobbelt/bootstrap,twbs/bootstrap,m5o/bootstrap,stanwmusic/bootstrap,stanwmusic/bootstrap,stanwmusic/bootstrap,tjkohli/bootstrap,kvlsrg/bootstrap,inway/bootstrap,GerHobbelt/bootstrap,kvlsrg/bootstrap,zalog/bootstrap,tjkohli/bootstrap,peterblazejewicz/bootstrap,m5o/bootstrap,coliff/bootstrap,coliff/bootstrap,zalog/bootstrap,inway/bootstrap,inway/bootstrap,twbs/bootstrap,peterblazejewicz/bootstrap