rust: Add highlight selector for doc comments (#15384)

## Release Notes:

- Added ability to style doc comments in Rust separately with
`comment.doc`
([#15322](https://github.com/zed-industries/zed/issues/15322)).


Just required adding another query to the highlights.scm file. Took
inspiration from the tree-sitter-rust repository
[here](https://github.com/tree-sitter/tree-sitter-rust/blob/master/queries/highlights.scm)

#### Doc comments customized in the theme are working now:

![grafik](https://github.com/user-attachments/assets/549f20d6-534c-4c3b-a317-3be6bc44352e)

#### Snippet from the .json theme file

![grafik](https://github.com/user-attachments/assets/da1d7e3d-b6a5-4ba5-9fef-047d69f9ea03)