rustup-{install,run,which,help,man}: add page (#10661)

pages/common/rustup-which.md

@@ -0,0 +1,13 @@
+# rustup which
+
+> Display which binary will be run for a given command managed by `rustup`.
+> Like `which`, but searches a Rust toolchain instead of `$PATH`.
+> More information: <https://rust-lang.github.io/rustup>.
+
+- Display the path to the binary in the default toolchain:
+
+`rustup which {{command}}`
+
+- Display the path to the binary in the specified toolchain (see `rustup help toolchain` for more information):
+
+`rustup which --toolchain {{toolchain}} {{command}}`