docs: add comprehensive backport workflow documentation

This commit adds detailed documentation for the automated backport
workflow and updates the contribution guidelines to reference it.

New documentation (docs/backport-workflow.md):
- Complete overview of the automated backport process
- Step-by-step usage instructions with examples
- Detailed explanation of workflow triggers and label format
- Technical details about workflow implementation
- Conflict resolution procedures and best practices
- Multiple backport scenarios and examples
- Comprehensive troubleshooting guide

Updated contribution guidelines (docs/code_contribution_guidelines.md):
- Replaced detailed backport instructions with brief overview
- Added reference to the new detailed documentation
- Keeps contribution guidelines focused and concise

The detailed documentation provides:
- How to use backport labels correctly
- What happens when labels are added before/after merge
- How the workflow validates branches and handles errors
- Step-by-step conflict resolution instructions
- Solutions for common problems and edge cases
- Examples of valid vs invalid label formats

This documentation ensures contributors and maintainers have clear
guidance on using the automated backport workflow effectively.

Author: ziggie1984