Rollup merge of #138061 - jieyouxu:target-reviewers, r=wesleywiser

triagebot: add a `compiler_leads` ad-hoc group

Intended for e.g. rolling reviewers for [adding new targets](https://forge.rust-lang.org/compiler/proposals-and-stabilization.html#targets).

rust-lang/rust-forge side doc update: rust-lang/rust-forge#815

r? ghost (for testing)

cc ``@davidtwco`` or ``@wesleywiser``

Author: compiler-errors