gdnative-doc 0.0.6

Documentation tool for gdnative
Documentation
- Add a reStructuredText backend ?
- Improve link resolution
  - Add a way to refer to other user-defined methods and structures, like
    ```markdown
    [`MyStruct::my_method`]
    ```
- Add more tests
- Add a cargo subcommand ? like `cargo godot-doc`...
- Support `#[register_with(...)]`