Update docs to match the new x.py defaults

[jyn514]

⚠️ This should not be merged until rust-lang/rust#73964 lands. ⚠️

• Update most pages to use build/test instead of build --stage 1
• Update pages to use build --stage 2 instead of build
• Remove debuginfo from the example config.toml, since the defaults now match the recommended value.

I probably missed some build without --stage and some wording, but I figure the leftovers can be fixed in a follow-up PR.

[JohnTitor]

I think https://rustc-dev-guide.rust-lang.org/building/bootstrapping.html also needs to update, e.g. stage 1 description, but we can follow-up later.

[jyn514]

rust-lang/rust#73964 was merged.

[JohnTitor]

Looks good!