feat: nobios mode, exercise docs & test cases, CI testing

[Ivans-11]

Summary

This PR introduces several major enhancements, including nobios mode support, exercises with documentation and test cases, and CI testing.

1. Nobios Mode Support

Enable the kernel to boot directly from QEMU's -bios none mode without an external SBI implementation.

Created a new sbi crate to replace the sbi-rt dependency.

Usage

# Traditional mode
cargo qemu --ch x

# Nobios mode
cargo qemu --ch x --nobios

2. Exercise Documentation and Test Cases

Implemented five basic experiments (ch3-ch6, ch8) following the rCore-Tutorial-Guide, with documentation and test cases.

Documentation is provided in exercise.md for each chapter, and test cases are adapted from rCore-Tutorial-Test.

Usage

# Run exercise
cargo qemu --ch x --exercise

3. CI Support

Merged CI #1 to support CI testing and eliminate compilation warnings.

Added the --ci parameter to separate CI testing from local testing modes:

• CI mode: Directly runs chX_usertest
• Local mode: Runs user_shell for manual test case input (convenient for local debugging)

Usage

# Run CI test
cargo qemu --ch x --exercise --ci