[red-knot] Move tuple-containing-Never tests to Markdown (#15402)

## Summary

See title.

Part of #15397

## Test Plan

Ran new Markdown test.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/type_properties/tuples_containing_never.md

@@ -0,0 +1,33 @@
+# Tuples containing `Never`
+
+A heterogeneous `tuple[…]` type that contains `Never` as a type argument simplifies to `Never`. One
+way to think about this is the following: in order to construct a tuple, you need to have an object
+of every element type. But since there is no object of type `Never`, you cannot construct the tuple.
+Such a tuple type is therefore uninhabited and equivalent to `Never`.
+
+In the language of algebraic data types, a tuple type is a product type and `Never` acts like the
+zero element in multiplication, similar to how a Cartesian product with the empty set is the empty
+set.
+
+```py
+from knot_extensions import static_assert, is_equivalent_to
+from typing_extensions import Never, NoReturn
+
+static_assert(is_equivalent_to(Never, tuple[Never]))
+static_assert(is_equivalent_to(Never, tuple[Never, int]))
+static_assert(is_equivalent_to(Never, tuple[int, Never]))
+static_assert(is_equivalent_to(Never, tuple[int, Never, str]))
+static_assert(is_equivalent_to(Never, tuple[int, tuple[str, Never]]))
+static_assert(is_equivalent_to(Never, tuple[tuple[str, Never], int]))
+
+# The empty tuple is *not* equivalent to Never!
+static_assert(not is_equivalent_to(Never, tuple[()]))
+
+# NoReturn is just a different spelling of Never, so the same is true for NoReturn
+static_assert(is_equivalent_to(NoReturn, tuple[NoReturn]))
+static_assert(is_equivalent_to(NoReturn, tuple[NoReturn, int]))
+static_assert(is_equivalent_to(NoReturn, tuple[int, NoReturn]))
+static_assert(is_equivalent_to(NoReturn, tuple[int, NoReturn, str]))
+static_assert(is_equivalent_to(NoReturn, tuple[int, tuple[str, NoReturn]]))
+static_assert(is_equivalent_to(NoReturn, tuple[tuple[str, NoReturn], int]))
+```

crates/red_knot_python_semantic/src/types.rs

@@ -4144,14 +4144,6 @@ pub(crate) mod tests {
         }
     }
 
-    #[test_case(Ty::Tuple(vec![Ty::Never]))]
-    #[test_case(Ty::Tuple(vec![Ty::BuiltinInstance("str"), Ty::Never, Ty::BuiltinInstance("int")]))]
-    #[test_case(Ty::Tuple(vec![Ty::Tuple(vec![Ty::Never])]))]
-    fn tuple_containing_never_simplifies_to_never(ty: Ty) {
-        let db = setup_db();
-        assert_eq!(ty.into_type(&db), Type::Never);
-    }
-
     #[test_case(Ty::BuiltinInstance("str"), Ty::BuiltinInstance("object"))]
     #[test_case(Ty::BuiltinInstance("int"), Ty::BuiltinInstance("object"))]
     #[test_case(Ty::BuiltinInstance("bool"), Ty::BuiltinInstance("object"))]