Please share your use cases of async_mqtt

[redboltz]

If you are using async_mqtt in your project, I would greatly appreciate it if you could leave a comment with an overview of how you are utilizing it. Even just a link to your project would be helpful.
Once a sufficient number of use cases are gathered, I plan to create an "Applications" section in the documentation to showcase them.

Your feedback will be a great motivation for me to continue developing async_mqtt!