Link to PDF version of course from first page (#1836)

Re
#1805 (review)

---------

Co-authored-by: Martin Geisler <[email protected]>

Author: max-heller

src/index.md

@@ -11,6 +11,8 @@ generics and error handling.
 > The latest version of the course can be found at
 > <https://google.github.io/comprehensive-rust/>. If you are reading somewhere
 > else, please check there for updates.
+>
+> The course is also available [as a PDF](comprehensive-rust.pdf).
 
 The goal of the course is to teach you Rust. We assume you don't know anything
 about Rust and hope to: