docs(child-device-agent): add deprecation notice

[reubenmiller]

Proposed changes

Add deprecation notice to the example but keep it as it is still referenced in older snapshots of the official thin-edge.io documentation (to prevent creating broken links).

A link to the new project, python-tedge-agent has also been added.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New example (non-breaking change which adds functionality)
• [ ] Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• [x] Documentation Update (if none of the other choices apply)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have signed the CLA (in all commits with git commit -s)
• [ ] I have added necessary documentation (if appropriate)

Further comments