[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/tuple.md

@@ -5,17 +5,14 @@
 ```py
 t = (1, 'a', 'b')
 
-a = t[0]
-b = t[1]
-c = t[-1]
-d = t[-2]
-e = t[4]        # error: [index-out-of-bounds]
-f = t[-4]       # error: [index-out-of-bounds]
+reveal_type(t[0])  # revealed: Literal[1]
+reveal_type(t[1])  # revealed: Literal["a"]
+reveal_type(t[-1])  # revealed: Literal["b"]
+reveal_type(t[-2])  # revealed: Literal["a"]
 
-reveal_type(a)  # revealed: Literal[1]
-reveal_type(b)  # revealed: Literal["a"]
-reveal_type(c)  # revealed: Literal["b"]
-reveal_type(d)  # revealed: Literal["a"]
-reveal_type(e)  # revealed: Unknown
-reveal_type(f)  # revealed: Unknown
+a = t[4]  # error: [index-out-of-bounds]
+reveal_type(a)  # revealed: Unknown
+
+b = t[-4]  # error: [index-out-of-bounds]
+reveal_type(b)  # revealed: Unknown
 ```