Tracking Issue for setting up codegen backend ping groups

[jieyouxu]

For cg_gcc and cg_clif.
Discussion: #t-infra > Triagebot ping groups for GCC and cranelift backends.

• Remove llvm and llvms triagebot ping aliases for icebreakers-llvm ping group Remove llvm and llvms triagebot ping aliases for icebreakers-llvm ping group #138735.
• Setup team marker-teams for cg_gcc and cg_clif. Add codegen-{cranelift,gcc} marker teams for custom codegen backend ping groups team#1685
• Configure r-l/r (this repo) triagebot.toml to accept the 2 new ping groups. Configure triagebot to accept codegen-{gcc,cranelift} ping groups and document ping groups and ping commands #138120
• Update rustc-dev-guide docs for the 2 new ping groups. Configure triagebot to accept codegen-{gcc,cranelift} ping groups and document ping groups and ping commands #138120

[jieyouxu]

Closing because this is entirely useless, since only team members and certain WG-* members can even use @rustbot ping xxx commands.