Update exercise boiler plate text to suggest working on the code locally

As pointed out in https://github.com/google/comprehensive-rust/pull/1288#issuecomment-1743131185, we're giving mixed signals about how the exercises should be solved: in the Rust Playground or locally with Cargo?

This is mostly a remnant of how the course has evolved: it started out being very focused on the Playground, but later we started recommending a more regular Rust setup.

We should update the boiler plate text to be more consistent and to encourage people to use a local installation.

To streamline things, we could include the same text (using the mdbook `{{#include }}` syntax) from all the exercises and thus ensure everything is consistent.