Slint reference: complete the documentation of data types, adjust layout types

[kud1ing]

The Slint reference currently only documents the primitive types: https://docs.slint.dev/latest/docs/slint/reference/primitive-types/

What is missing:

• Arrays are only documented in the guide
• Structs and enums are only documented in the guide
• Tuples are only mentioned in the type mappings?
• Anonymous objects are only mentioned in the type mappings?

I suggest to document all data types in the Slint reference.

Regarding layout I also noticed:

• Basic Layout talks about *Layout types, while Widget Layouts writes about *Box types. Do really both exist?

[NigelBreslaw]

These sound good. Feel free to make a PR with some suggested changes. The docs are mostly in markdown. Otherwise I can have a look at these as part of future doc improvements.

[kud1ing]

I am not sure I will find the time, sorry.