Document `createMiddleware` for Next.js

[qw-in]

@qw-in I noticed that createMiddleware is defined here but seems to be undocumented on the website.
Should it be there? Should it be deprecated here?

Originally posted by @wooorm-arcjet in arcjet/arcjet-js#5038 (comment)

[blaine-arcjet]

FWIW, we've found that most users need to move away from createMiddleware pretty quickly. It was a nice way to get setup quickly, but Next.js doesn't make composing middleware easy—as soon as you need to apply 2 middlewares, you usually need to drop out of this API and use aj.protect() directly.