Update mqtt.md for buttons and motion sensors

[dvaandrager]

Include extra information about the button and motion sensor topics that is not easily found elsewhere and should be included here for completeness.

[DedeHai]

thanks. you should really put this into knowledge base, the documents in this folder are all 5 years old, outdated and not maintained.

[dvaandrager]

thanks. you should really put this into knowledge base, the documents in this folder are all 5 years old, outdated and not maintained.

I got here from the edit icon on the MQTT page at https://kno.wled.ge/interfaces/mqtt/ so I thought this was the most current documentation? Is there somewhere else I should be looking and/or updating?

[DedeHai]

sorry my bad, I thought this was some source code document, but it actually IS the knowledge base, so all good