Auto merge of #5971 - dwijnand:one-architecture, r=alexcrichton
Auto merge of #5971 - dwijnand:one-architecture, r=alexcrichton

Migrate some architecture notes from the wiki

Merge https://github.com/rust-lang/cargo/wiki/Cargo's-architecture into
the ARCHITECTURE.md in the repo.

Fixes #5528

vojtechkral/cargo,tshepang/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,vojtechkral/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,tshepang/cargo,vojtechkral/cargo,rustyhorde/cargo,vojtechkral/cargo,rust-lang/cargo,tsurai/cargo,rust-lang/cargo,rust-lang/cargo,tsurai/cargo,tsurai/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo