Better syntax highlighting for rust code

The syntax highlighting we're using in code examples doesn't clearly distinguish many of the syntax elements of rust code. This makes it far less readable.

For example, this is our docs:

And this is vscode:

Start by locating the syntax-highlighting configuration used for the documentation's Rust code examples and compare its output with the provided VS Code example. Determine which Rust syntax elements are currently indistinguishable; done means the documentation examples are visibly clearer and correctly highlight the relevant elements.

Written by the indexing model from the issue text.