docs: enhance Rust guide

[no1wudi]

docs: enhance Rust guide with CPU optimization instructions

Summary

• Added a section to the Rust guide that explains how to use the RUSTFLAGS environment variable to specify the target CPU for optimization.
• Provides examples and instructions for setting RUSTFLAGS for different CPU models, such as Cortex-M33 and Cortex-M55, to generate performance-optimized code.

Impact

• Enhances the developer's ability to produce more efficient Rust applications on NuttX by leveraging CPU-specific optimizations.
• No runtime or functional changes; only documentation improvements to guide users in optimizing their build configurations for specific hardware.

Test

local document build

[nuttxpr]

[Experimental Bot, please feedback here]

Yes, this PR appears to meet the NuttX requirements, although it could be slightly improved. Here's a breakdown:

Strengths:

• Clear Summary: The summary explains the "why," "what," and "how" of the change. While it doesn't link a specific issue, this is understandable for a documentation-only change.
• Impact is Well-Defined: The impact section clearly addresses most points, highlighting that this is a documentation-only change with no functional impact.
• Testing Justification: The author confirms local document build testing, which is appropriate for a documentation change. Including the actual build output (even if successful) would further strengthen this section.

Areas for Improvement:

• Explicit "NO" in Impact: While the impact section is good, explicitly stating "NO" for areas without impact (e.g., "Impact on build: NO") would enhance readability and clarity.
• More Complete Testing Information: While a local build is sufficient, specifying the OS and build tools used (as requested in the template) would make the testing information more robust. Example: "Build Host: macOS Ventura, clang version X.Y.Z". Also, showing some output, even if just a successful build confirmation, would be helpful.
• Consider a small example in the documentation itself: A small, runnable example in the updated documentation showcasing the RUSTFLAGS usage would significantly improve the documentation's practical value.

Conclusion:

The PR is generally well-structured and meets the core requirements. Addressing the minor improvements mentioned above would make it even stronger and easier to review.