docs: Add some App Router FAQs

[leerob]

Based on questions I've seen in issues and discussions.

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Secrets	0   1   0   0	View in Orca

[mayank1513]

Great!

[GabenGar]

It looks like the redirect functions are hardcoded to 307/308 redirects. How do you do 301/302/303 redirects then?

[leerob]

Why does Next.js use 307 and 308? Traditionally a 302 was used for a temporary redirect, and a 301 for a permanent redirect, but many browsers changed the request method of the redirect to GET, regardless of the original method. For example, if the browser made a request to POST /v1/users which returned status code 302 with location /v2/users, the subsequent request might be GET /v2/users instead of the expected POST /v2/users. Next.js uses the 307 temporary redirect, and 308 permanent redirect status codes to explicitly preserve the request method used.

From the docs 🙏

[GabenGar]

Yeah but there might be use cases for when you want to change the method to GET and 301/302/303 are go-to SEO redirects (SEO doesn't operate outside of GET requests anyway). I have a suspicion some search engines can choke on 307/308 by assuming these are invalid codes.