rustup-{set,self,completions,doc,override,component}: add page (#10729)

* rustup set: add page

* rustup self: add page

* rustup completions: add page

* rustup doc: add page

* rustup override: add page

* rustup component: add page

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Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/rustup-completions.md

@@ -0,0 +1,8 @@
+# rustup completions
+
+> Generate shell completions for `rustup` and `cargo`.
+> More information: <https://rust-lang.github.io/rustup>.
+
+- Print the completion script to `stdout`:
+
+`rustup completions {{bash|elvish|fish|powershell|zsh}} {{rustup|cargo}}`