Should close #605

Notes on implementation:

• section needs to be consumed as well as constructed, therefore I added a constructor after the type definition. This works without any issues, as tested for this case but also from prior experience.
• To avoid confusion with extraData, I chose the name moreFields.
• If the additional key moreFields was another parameter ('moreFields), then section would need to be typed as section('item, 'moreFields) which necessitates specifying 'moreFields when creating section - if the key is not used. To avoid such unnecessary complexity, I chose to define a moreFields type instead and added a function to cast anything as that type.

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Looks good to me.

I would just find a more explicit name for moreFields, maybe something like additionalSectionData or just additionalData

Since additional fields will be useful only for data relevant to the section (such as title as in the example given in #605), I thought sectionData might be a better name. What do you think @MoOx ?

I also remembered to update documentation