Add CTA at the end of the walkthrough

The walkthrough currently ends after the user has downloaded the CLI. This leaves the user on a challenge page with no obvious steps forward.

It would be great if the walkthrough could be extended to support the user through downloading the challenge:

```
Great! Now you can download the challenge.

Run this command in your terminal to download the challenge:

> mentorcism download hello-world --track=c

Have you downloaded the challenge?

- Yes
- No
```

```
Open the challenge in your favourite editor and solve it!

When you're ready, submit your solution with the following command:

> mentorcism submit mentorcism/c/hello-world

All done?

- Yes
- No
```

Thoughts welcome on possible copy and associated steps