Update migrating from Gatsby docs. (#17636)
Update migrating from Gatsby docs. (#17636)

Addresses remaining comments from https://github.com/vercel/next.js/pull/17491.
azukaru/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js