Update docs to show how to typecheck next.config.js (#25240)
Update docs to show how to typecheck next.config.js (#25240)



## Documentation

Partially resolves #5318

- [x] Make sure the linting passes

JeromeFitz/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,flybayer/next.js,zeit/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js