docs: Add FAQ section?

[taiki-e]

It might be nice with a section mentioning the often overlooked standard library APIs such as Option::as_pin_mut (e.g., https://twitter.com/fasterthanlime/status/1377229099622662146, hyperium/tonic#1377, tower-rs/tower#316 (comment)), Pin::as_mut (e.g., #345), Pin::set (e.g., https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/.60Pin.60.20used.20for.20encapsulation/near/300322988, tower-rs/tower#323 (comment)), etc., and cases like #226.

Related discussion: #89 (comment)

[taiki-e]

Q. Do I require to learn about obscure concepts like structural pinning when using this crate?

A. No, pin-project is intended to be safe to use without understand structural pinning.

https://x.com/_taiki_e/status/1846184768683364790