Add Wikipedia challenge at the intro of git lesson

At the introduction of our Git lesson we list the advantages of using one version control system. Should be easy to students understand the advantages of using it but maybe we could propose a small challenges for students (what this PR does).

## Why Wikipedia

I expect that 100% of our students have at least one read one article from Wikipedia and not be afraid of it.  So asking for some information that they could "easily" find will demonstrate the advantages of version control and be consistent with the last part of our intro:

> No matter which version control system you use, the most important thing to learn is not the details of their more obscure commands, but the workflow that they encourage.

## Why not GitHub

I expect that at least half of our students never heard about GitHub before and ask for any basic information could be harm.
I would drop this change and introduce Wikipedia down just before the challenge.