Keep indentation in doc comments and simplify

[oxalica]

Leading spaces is significant for Markdown and we should not blindlessly trim them, which may break Markdown lists or code blocks.

Here we strip the minimal indentation of all non-empty doc comment lines, so we can both keep Markdown semantics and avoid unnecessary leading spaces immediately after the /// token.

Eg.

use utoipa::{OpenApi, ToSchema};

/// Summary
///
/// List:
/// - foo
///   bar
/// - baz
#[derive(ToSchema)]
struct Typ {}

The bar line is indented and should continue the previous paragraph in the first unordered list element foo. But if stripping the leading whitespaces, bar will start a new paragraph, which is incorrect.

[juhaku]

This is super 💯 Same drill could you mark the changes to the CHANGELOG in utoipa-gen since it touches it's functionality.