move v2 => v3 migration docs to separate page

Created by: cvrebert

The migration docs currently take up a sizable chunk of the Getting Started page, but new users don't care about migration, so it clutters the Getting Started page for them. Plus, we're well into the swing of v3 now, so it seems fair to put less emphasis on migration than before.

We should still link to the new Migration page from the Getting Started page though.