Copyright notice in code snippets

[hurryabit]

Most code snippets in the course show a copyright header of the form

// Copyright 20xx Google LLC
// SPDX-License-Identifier: Apache-2.0

This is a problem since it's visual noise that takes up valuable screen space.

Some code blocks don't show this header though. My preliminary investigation suggests that blocks opened by only

```rust

don't show the copyright header whereas those opened by more complex fences, such as

```rust,editable

do show it.

[mgeisler]

Yes, I can reproduce this. The problem is that mdbook does not support hiding code when the text areas are editable. It knows to strip away the leading #, but that is it.

Basically, hiding lines in the output is not compatible with also giving the user access to edit the code block.

[mgeisler]

The problematic PR is #3097.