docs: Rename hello_rust to rust/baremetal to reflect the actual codebase

[no1wudi]

Summary

• The hello_rust example has been moved to rust/baremetal to better reflect its purpose as a bare-metal Rust application.
• The move also helps in organizing the Rust examples more clearly, separating them into categories based on their environment (bare-metal vs. non-bare-metal).
• A new directory structure has been created under rust/baremetal, and the index.rst file has been updated to describe the baremetal example in detail.
• The top-level rust/index.rst file has been created to include the new rust/baremetal directory in the toctree.

Impact

• The new file structure and documentation improvements will help new contributors and users understand the purpose and setup of the example more easily.
• This change aligns the documentation with the actual codebase, enhancing the overall maintainability and clarity of the project.

Testing

GitHub CI and local document build