chapters/software-stack: Add solution for `main_printf.c`

[teodutu]

Prerequisite Checklist

• [x] Read the contribution guidelines regarding submitting new changes to the project;
• [x] Tested your changes against relevant architectures and platforms;
• [x] Updated relevant documentation (if needed).

Description of changes

The solution now has the complete solution with comments both for puthcar_() and for using sprintf() + write() instead of printf().