[chore] adds a clean-images Makefile target

[puckpuck]

Changes

It adds a clean-images target, removing all demo images only. This is much less impactful to someone's system than running docker system prune—a when you are having issues with image cache.

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[julianocosta89]

This is great!