Add: markdown code block langs

[mattmundell]

What:

Add languages to ``` blocks in Markdown files in the Rust tree.

Why:

Enables syntax highlighting.

[Tehforsch]

Unfortunately, the rs annotation in the rust markdown blocks still causes our doctests not to run. We currently don't have a mechanism to ignore these, since we directly include the md files in the rust modules. If you replace them, with rust it'll be GTM