Github renders #include as a literal # include

In our exercises, on Github, the solutions look like:

```
{{#include ../../exercise-solutions/green-yellow/src/bin/step3.rs:3:13}}
```

It would be nice if there was some Github-compatible syntax for including files into a markdown file - but I suspect there isn't. We would then have to decide if we just want to copy-paste the solution code instead.