Detect code block marker in docs without "base indentation"

[ojeda]

What it does

In the Linux kernel we had some documentation examples added with triple backquotes at the end without indentation.

Such a lint would find a few hits, e.g. there is currently one in core.

Possibly related, at least implementation-wise: #15024 and #15025.

Advantage

• More consistency in docs.
• Less time spent by reviewers.

Drawbacks

No response

Example

/// ```
/// fn f() {}
///```

Should be written as:

/// ```
/// fn f() {}
/// ```