Detect code block contents without "base indentation" in docs

[ojeda]

What it does

In many projects, it is likely that no indentation is not indented.

Such a lint would find a few hits, e.g. there are currently a few in stdarch.

Having said that, when/if rustfmt stabilizes formatting code blocks in documentation, this lint may not be needed anymore.

Possibly related, at least implementation-wise: #15023 and #15025.

Advantage

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

Drawbacks

No response

Example

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

Should be written as:

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