[red-knot] Add diagnostic for class-object access to pure instance variables (#16036)

## Summary

Add a diagnostic if a pure instance variable is accessed on a class object. For example

```py
class C:
    instance_only: str

    def __init__(self):
        self.instance_only = "a"

# error: Attribute `instance_only` can only be accessed on instances, not on the class object `Literal[C]` itself.
C.instance_only
```


---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/red_knot_python_semantic/resources/mdtest/annotations/string.md

@@ -116,8 +116,8 @@ MyType = int
 class Aliases:
     MyType = str
 
-    forward: "MyType"
-    not_forward: MyType
+    forward: "MyType" = "value"
+    not_forward: MyType = "value"
 
 reveal_type(Aliases.forward)  # revealed: str
 reveal_type(Aliases.not_forward)  # revealed: str