Add image base path configuration option (#23115)
Add image base path configuration option (#23115)

I similarly [couldn't find this info](https://github.com/vercel/next.js/issues/19711#issuecomment-750414110) in the docs about configuring the image path when using an app `basePath`, so thought I'd add it.
zeit/next.js,azukaru/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,flybayer/next.js,flybayer/next.js