Update syntax with examples, callables and wording

[maxfischer2781]

This pull request includes:

• categories for constructs (e.g. Containers, Callable, ...),
• example values to most types, e.g. foo = (1, 'two', 3.0) for foo: "(int, str, float)".
• new (a: int, *float, b: bool, **str)Callable syntax for variadic and named arguments.

Since the Callable syntax is a proposal, it is currently in .. hint:: blocks.

[maxfischer2781]

I've fixed the typo of the first section and renamed *R to iter R on all occurrences I found.

👍 on adding content tabs, but I recommend doing that in a separate PR.