[rust-lang/rust] rustdoc-json: Move `#[doc` attribute out of `Attribute::Other` to own well-typed variant

### ROOT CAUSE The `#[doc]` attribute is currently stored in `Attribute::Other` as a string, which is not ideal because it does not have its own variant and might be treated as a regular attribute. This can lead to loss of information or difficulty in processing, especially since `#[doc]` is a special attribute for documentation. ### CODE FIX Change the `Attribute` enum in the rustdoc_json crate to include a `Doc` variant that holds the entire string of the `#[doc]` attribute. Then, change the attribute parsing function to use the `Doc` variant for attributes with name `doc`. This ensures that the `#[doc]` attribute is properly typed and handled separately. Specifically, change the `Attribute` enum from: ```rust pub enum Attribute { Other(String), // ... other variants ... } ``` to: ```rust pub enum Attribute { Doc(String), Other(String), // ... other variants ... } ``` Then, change the function that parses attributes (e.g., `parse_attribute`) to: ```rust pub fn parse_attribute(attr: &Attribute_) -> Attribute { if attr.name == "doc" { Attribute::Doc(attr.to_string()) } else { // ... existing code for other attributes ... } } ``` Also, update any code that uses the `Attribute` enum to handle the new `Doc` variant appropriately.