bpo-30466: Add brief explanation of classes to tutorial

[treyhunner]

This could certainly use more work. A "what are classes" or "why classes" section would be great, especially one that explains the various class terminology.

We could alternatively or additionally link to external resources that explain classes at a beginner-friendly level.

The "A First Look at Classes" section could also probably be moved up the page or it could be noted that beginners can skip over the first couple sections before it.

[mention-bot]

@treyhunner, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @avassalotti and @serhiy-storchaka to be potential reviewers.

[the-knights-who-say-ni]

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[Mariatta]

I think the change here looks good. Raymond indicated in the issue that he wants to review this, so I'll wait for his feedback :)
Thanks @treyhunner

[DimitrisJim]

Reading this, wouldn't the flow be more natural if this was the second paragraph and not the first?

[Mariatta]

No, I think starting with a brief intro of what classes are, like this paragraph here, is good.

[Mariatta]

Thanks @treyhunner and congrats on your first contribution to CPython 🎉

Can you please backport this to the 3.6 branch? You can use cherry_picker.

[treyhunner]

Thanks @Mariatta!

I just open a pull request to backport the changes: #2700.