Prep to release v0.42.0 (#2003)

* Prep to release v0.42.0

* Remove test_context reference from example

* standalone_crate for doc tests because codegen is expensive

* fmt

subxt/src/client/online_client.rs

@@ -214,7 +214,7 @@ impl<T: Config> OnlineClient<T> {
     ///
     /// # Example
     ///
-    /// ```no_run
+    /// ```rust,no_run,standalone_crate
     /// # #[tokio::main]
     /// # async fn main() {
     /// use subxt::{ OnlineClient, PolkadotConfig };