Implement Black's rules around newlines before and after class docstrings (#6209)

## Summary

Black allows up to one blank line _before_ a class docstring, and
enforces one blank line _after_ a class docstring. This PR implements
that handling. The cases in
`crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/class_definition.py`
match Black identically.

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/class_definition.py

@@ -43,3 +43,52 @@ class TestTrailingComment2: # trailing comment
     pass
 
 
+class Test:
+    """Docstring"""
+
+
+class Test:
+    # comment
+    """Docstring"""
+
+
+class Test:
+    """Docstring"""
+    x = 1
+
+
+class Test:
+    """Docstring"""
+    # comment
+    x = 1
+
+
+class Test:
+
+    """Docstring"""
+
+
+class Test:
+    # comment
+
+    """Docstring"""
+
+
+class Test:
+
+    # comment
+
+    """Docstring"""
+
+
+class Test:
+
+    """Docstring"""
+    x = 1
+
+
+class Test:
+
+    """Docstring"""
+    # comment
+    x = 1