Rollup merge of #76631 - jyn514:x.py-setup, r=Mark-Simulacrum

Add `x.py setup`

Closes #76503.

- Suggest `x.py setup` if config.toml doesn't exist yet
- Prompt for a profile if not given on the command line
- Print the configuration that will be used
- Print helpful starting commands after setup
- Link to the dev-guide after finishing

diff --cc src/bootstrap/CHANGELOG.md
Simple merge

diff --cc src/bootstrap/bin/main.rs
Simple merge

diff --cc src/bootstrap/config.rs
index 53fef7a838df62dc11152491dea44a32ac0d1719,46a9d989652e93af5405d6728df9d1baafd8b0a0..b14746dabb93a83eff5cdf9caea130b602668bd9
--- 1/src/bootstrap/config.rs
--- 2/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@@ -71,8 -71,9 +71,10 @@@ pub struct Config 
      pub on_fail: Option<String>,
      pub stage: u32,
      pub keep_stage: Vec<u32>,
 +    pub keep_stage_std: Vec<u32>,
      pub src: PathBuf,
+     // defaults to `config.toml`
+     pub config: PathBuf,
      pub jobs: Option<u32>,
      pub cmd: Subcommand,
      pub incremental: bool,

diff --cc src/bootstrap/flags.rs
Simple merge