Link "about rustup" to the rustup book
&nbsp;&middot;&nbsp;
<a href="https://rust-lang.github.io/rustup-components-history/">component availability</a>
&nbsp;&middot;&nbsp;
- <a href="https://github.com/rust-lang/rustup">about rustup</a>
+ <a href="https://rust-lang.github.io/rustup/">about rustup</a>
</p>
<script src="rustup.js"></script>
