Contracts update doc.rs metadata (#3241)

Adding Rust metadata for doc
see https://docs.rs/about/metadata

---------

Co-authored-by: Alexander Theißen <alex.theissen@me.com>

substrate/frame/contracts/uapi/Cargo.toml

@@ -23,6 +23,9 @@ scale = { package = "parity-scale-codec", version = "3.6.1", default-features =
 [target.'cfg(target_arch = "riscv32")'.dependencies]
 polkavm-derive = '0.5.0'
 
+[package.metadata.docs.rs]
+default-target = ["wasm32-unknown-unknown"]
+
 [features]
 default = ["scale"]
 scale = ["dep:scale", "scale-info"]