Derive Macro ConfigDoc

#[derive(ConfigDoc)]
{
    // Attributes available to this derive:
    #[config_doc]
}

This proc macro recursively extracts rustdoc comments from the fields in a struct and generates a method that produces docs for each field.

Unlike rustdoc, this tool focuses solely on extracting rustdoc for struct fields, without methods.

Example:

#[derive(ConfigDoc)]
pub struct Foo {
  /// Description for `a`.
  a: i32,

  #[config_doc(nested)]
  b: Bar,

  #[config_doc(omitted)]
  dummy: (),
}

The #[config_doc(nested)] attribute indicates that the field is a nested config that will be documented in a separate section. Fields marked with #[config_doc(omitted)] will simply be omitted from the doc.

Here is the method generated by this macro:

impl Foo {
    pub fn config_docs(name: String, docs: &mut std::collections::BTreeMap<String, Vec<(String, String)>>)
}

In test_example_up_to_date, we further process the output of this method to generate a markdown in src/config/docs.md.