[red-knot] mdtest suite: formatting and cleanup (#13806)

Minor cleanup and consistent formatting of the Markdown-based tests.

- Removed lots of unnecessary `a`, `b`, `c`, … variables.
- Moved test assertions (`# revealed:` comments) closer to the tested
object.
- Always separate `# revealed` and `# error` comments from the code by
two spaces, according to the discussion
[here](https://github.com/astral-sh/ruff/pull/13746/files#r1799385758).
This trades readability for consistency in some cases.
- Fixed some headings

crates/red_knot_python_semantic/resources/mdtest/binary/integers.md

@@ -1,21 +1,14 @@
-## Binary operations on integers
+# Binary operations on integers
 
 ## Basic Arithmetic
 
 ```py
-a = 2 + 1
-b = a - 4
-c = a * b
-d = c // 3
-e = c / 3
-f = 5 % 3
-
-reveal_type(a)  # revealed: Literal[3]
-reveal_type(b)  # revealed: Literal[-1]
-reveal_type(c)  # revealed: Literal[-3]
-reveal_type(d)  # revealed: Literal[-1]
-reveal_type(e)  # revealed: float
-reveal_type(f)  # revealed: Literal[2]
+reveal_type(2 + 1)  # revealed: Literal[3]
+reveal_type(3 - 4)  # revealed: Literal[-1]
+reveal_type(3 * -1)  # revealed: Literal[-3]
+reveal_type(-3 // 3)  # revealed: Literal[-1]
+reveal_type(-3 / 3)  # revealed: float
+reveal_type(5 % 3)  # revealed: Literal[2]
 ```
 
 ## Division by Zero
@@ -33,16 +26,19 @@ subclass; we only emit the error if the LHS type is exactly `int` or `float`, no
 
 ```py
 a = 1 / 0  # error: "Cannot divide object of type `Literal[1]` by zero"
-b = 2 // 0  # error: "Cannot floor divide object of type `Literal[2]` by zero"
-c = 3 % 0  # error: "Cannot reduce object of type `Literal[3]` modulo zero"
-d = int() / 0  # error: "Cannot divide object of type `int` by zero"
-e = 1.0 / 0  # error: "Cannot divide object of type `float` by zero"
-
 reveal_type(a)  # revealed: float
+
+b = 2 // 0  # error: "Cannot floor divide object of type `Literal[2]` by zero"
 reveal_type(b)  # revealed: int
+
+c = 3 % 0  # error: "Cannot reduce object of type `Literal[3]` modulo zero"
 reveal_type(c)  # revealed: int
+
+d = int() / 0  # error: "Cannot divide object of type `int` by zero"
 # TODO should be int
 reveal_type(d)  # revealed: @Todo
+
+e = 1.0 / 0  # error: "Cannot divide object of type `float` by zero"
 # TODO should be float
 reveal_type(e)  # revealed: @Todo