Issue #1513 Document how to write child device config management agent

[albinsuresh]

Proposed changes

Document how to write child device config management agent

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

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

[didier-wenzek]

Clear writing. Might be improved with some more context in the introduction, though.

• The spell checker report typos.
• mdbook checker reports markdown issues.
• The SUMMARY.md must be updated.

[didier-wenzek]

Approved. A really nice documentation, clear and pleasant reading.