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

src/building/new-target.md

@@ -4,8 +4,13 @@ These are a set of steps to add support for a new target. There are
 numerous end states and paths to get there, so not all sections may be
 relevant to your desired goal.
 
+See also the associated documentation in the
+[target tier policy][target_tier_policy_add].
+
 <!-- toc -->
 
+[target_tier_policy_add]: https://doc.rust-lang.org/rustc/target-tier-policy.html#adding-a-new-target
+
 ## Specifying a new LLVM
 
 For very new targets, you may need to use a different fork of LLVM