docs: Remove obsolete/redundant backend document

Author: Turbo87

docs/ARCHITECTURE.md

@@ -18,8 +18,8 @@ Documentation about the codebase appears in these locations:
 
 The backend of crates.io is written in Rust. Most of that code lives in the _src_ directory. It
 serves a JSON API over HTTP, and the HTTP server interface is provided by the [axum][] crate and
-related crates. More information about the backend is in
-[`docs/BACKEND.md`](https://github.com/rust-lang/crates.io/blob/master/docs/BACKEND.md).
+related crates. More information about backend development is in
+[`docs/CONTRIBUTING.md`](https://github.com/rust-lang/crates.io/blob/master/docs/CONTRIBUTING.md).
 
 [axum]: https://crates.io/crates/axum