docs: Add more schema docs around lists

[WolffRuoff]

This PR adds more documentation around lists in the schema basics documentation page. More specifically it adds the following:

• Explicitly state that nested lists work as defined here in the GraphQL spec.
• Add a table with examples for list nullability. I've found that list nullability is often a hurdle for new GraphQL developers and a table with examples will hopefully make this concept a lot easier to understand

This is my first PR here so please let me know if I did anything wrong or how I can improve to better match the documentation style 😄.

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