Add preprocessing step to automate numbering in tutorial introduction

[bennibolm]

Right now, when someone wants to add a new tutorial, he has to update all following numbers within the introduction. This is done for instance in this PR.

The new preprocessing step automates the numbering.

[github-actions]

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[bennibolm]

I don't really know if this change is wanted but for me, it was annoying to update all the numbers when adding a new tutorial.
Also, the numbering within the tutorials is automatic as well using the preprocessing step from here.

[sloede]

Good idea! I've left some comments. It would also be great to get a feedback from @ArseniyKholod, since he's also been active writing tutorials...

[sloede]

LGTM and can be merged once @ArseniyKholod signs off as well 🙂

Thank you for the nice improvement of developer convenience 👍

[ArseniyKholod]

Thanks! Really good idea, it was annoying to change all indexes manually. LGTM!

[bennibolm]

Oh, just realized (again) that auto-merge doesn't work here. Please be so nice and merge it @sloede 😄