Rollup merge of #135992 - madsmtm:new-target-docs, r=jieyouxu

Improve documentation when adding a new target

rust-lang/rust#133631 (comment) shows that it can be a bit difficult process-wise to add a new target.

I've added a bit of text to the docs, suggesting that users add the target defintion/spec first, and later work on `std` support.

I also found that we have two places where we document how to add a new target. I've linked these for now, but they should probably be merged somehow in the future.

`@rustbot` label A-docs
r? compiler
CC `@workingjubilee` who's worked a lot on target specs IIRC.

Author: matthiaskrgr