Improve documentation when adding a new target https://github.com/rust-lang/rust/pull/133631#issuecomment-2607877936 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.