Cheerpj interoperability tutorial

[theodoravraimakis]

No description provided.

[epanholz]

I think we should restructure this tutorial a bit to make sure people understand it fully once completed. That's why I recommended so many changes. The content overall looks good, just the order of things could be improved and some code needs a few more explanations.
I am also unsure if the natives-guide commit is supposed to be part of this branch or not, please double check that.

[epanholz]

Nearly there. Just a few more structural changes. Overall the tutorial looks really good already. Good job.

Also please double check if the first commit about the native methods guides is supposed to be in this PR. You can also --amend to your previous commit, instead of making a new one again.

[GabrielaReyna]

I can do a quick review on formatting and styling once Elisabeth's changes are fulfilled.

[epanholz]

This looks good to me now besides the missing line in the JS code.
@GabrielaReyna please also have a look before merging. I would really appreciate your input on the tutorial to make sure it's all easy to understand and clear.

[GabrielaReyna]

Same as the previous PR, please correct the capitalisation on subtitles.