Add migration guide from Gatsby

[leerob]

Feature request

As a user of [x] framework/language, I'd love to see an easy path towards migrating to Next.js

Describe the solution you'd like

Documentation around common providers and how exactly you can switch to Next.js from them.

Additional context

Let's start with a few of the most popular and see what others in the community would get value from.

• Create React App (Add migration guide from Create React App #17454)
• Gatsby
• Vanilla React (Add migration guide from vanilla React #17455)

[Avi98]

@leerob can I take this issue

[leerob]

@Avi98 Hey! I've already got a PR in progress for this one 👍

[lfades]

Added on #17491