Deeper dive into Questions in docs

Hi!

I was setting up a pretix instance and had a really hard time figuring out how to add custom fields to my ticket. https://github.com/pretix/pretix-docs/blob/main/docs/tutorial/event.md?plain=1 doesn't cover Questions, and I don't see a guide to questions in https://docs.pretix.eu/guides/

I ran into a lot of confusion about why I couldn't add a required field to a ticket. These docs - while comprehensive and detailed - was making my head spin. I eventually set up a non-production test page and just clicked around forever until I figured it out. (for context, we needed different questions on different ticket types, for example 18+ tickets require acknowledgment of a waiver and a question about what type of vehicle you're bringing, while U18 requires guardian acknowledgement of a different type of waiver and information about the guardian themselves).

Would you accept a PR that adds some information about Questions? I was thinking:

• A new section in the Event tutorial covering how to add a Question to a product and marking it as required
• A new subpage of the Product guide https://docs.pretix.eu/guides/products/ that covers setting up questions and attaching them to products

Let me know if I'm off base here, or if this wouldn't be accepted. If you're into the idea, I'm happy to submit a PR. Thank you!

Start with docs/tutorial/event.md and the product guide at docs.pretix.eu/guides/products/. Document how to create Questions, mark them as required, and attach different questions to products or ticket types; done means a newcomer can follow the guide for these setups without relying on trial and error.