[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/subscript/string.md

@@ -5,19 +5,16 @@
 ```py
 s = 'abcde'
 
-a = s[0]
-b = s[1]
-c = s[-1]
-d = s[-2]
-e = s[8]        # error: [index-out-of-bounds] "Index 8 is out of bounds for string `Literal["abcde"]` with length 5"
-f = s[-8]       # error: [index-out-of-bounds] "Index -8 is out of bounds for string `Literal["abcde"]` with length 5"
+reveal_type(s[0])  # revealed: Literal["a"]
+reveal_type(s[1])  # revealed: Literal["b"]
+reveal_type(s[-1])  # revealed: Literal["e"]
+reveal_type(s[-2])  # revealed: Literal["d"]
 
-reveal_type(a)  # revealed: Literal["a"]
-reveal_type(b)  # revealed: Literal["b"]
-reveal_type(c)  # revealed: Literal["e"]
-reveal_type(d)  # revealed: Literal["d"]
-reveal_type(e)  # revealed: Unknown
-reveal_type(f)  # revealed: Unknown
+a = s[8]  # error: [index-out-of-bounds] "Index 8 is out of bounds for string `Literal["abcde"]` with length 5"
+reveal_type(a)  # revealed: Unknown
+
+b = s[-8]  # error: [index-out-of-bounds] "Index -8 is out of bounds for string `Literal["abcde"]` with length 5"
+reveal_type(b)  # revealed: Unknown 
 ```
 
 ## Function return