Detect extra "base indentation" in Rust code blocks in docs

What it does

For Rust code blocks, and for code blocks in many other languages, it is very likely that extra indentation is not indented if it is applied across the entire contents of the code block, e.g. an extra space on all lines (or even just all non-empty lines).

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: https://github.com/rust-lang/rust-clippy/issues/15023 and https://github.com/rust-lang/rust-clippy/issues/15024.

Advantage

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

Drawbacks

No response

Example

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

Should be written as:

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

Start with the Rust documentation code-block example in this issue and review the related Clippy issues 15023 and 15024 for implementation context. Done means a lint detects uniform extra indentation in Rust documentation code blocks and reports the example shown, while accounting for the stated possibility that rustfmt may make it unnecessary.