examples/rust: Rename hello_rust to rust/baremetal

[no1wudi]

Summary:

• Renamed hello_rust example to rust/baremetal to better reflect its purpose
• Updated file headers and Makefile paths
• Maintained all existing Kconfig options and functionality
• Prepares for future Rust development patterns where Cargo will be the mainstream approach, with baremetal demonstrating traditional build method
• Creates clearer structure for Rust examples as we expand Rust support

Impact:

• No functional changes to the example itself
• Better organization of Rust examples under examples/rust/
• Existing configurations using this example will continue to work
• Build system will now look for the example in the new location

Testing:

• Confirmed Kconfig options remain unchanged
• GitHub CI passed and local build tested

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR appears to meet the NuttX requirements. The summary clearly explains the "why," "what," and "how" of the change. The impact section addresses all relevant points, noting no user-facing changes and describing the build system impact. The testing section confirms verification on local setup and mentions successful CI runs, although ideally, specific host/target details and before/after logs would be included as per the template. While the logs are missing, the rest of the information provided suggests sufficient testing.