rustup: update page, rustup-{show,update,check,default,toolchain,target}: add page (#10642)

pages/common/rustup-update.md

@@ -0,0 +1,12 @@
+# rustup update
+
+> Update Rust toolchains and `rustup` itself (if not installed using a package manager).
+> More information: <https://rust-lang.github.io/rustup>.
+
+- Update all installed toolchains and `rustup`:
+
+`rustup update`
+
+- Install or update a specific toolchain (see `rustup help toolchain` for more information):
+
+`rustup update {{toolchain}}`