Add examples to tedge config list --doc

[didier-wenzek]

Proposed changes

Add examples to:

• mqtt.topic_root
• mqtt.device_topic_id

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

#2462

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (190b6c9) 75.8% compared to head (403b515) 75.8%.
Report is 5 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
.../tedge_config/src/tedge_config_cli/tedge_config.rs	81.1% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
379	0	3	379	100	57m57.498999999s

[reubenmiller]

Approved