docs(mqtt): observer follow-ups

[MrAlders0n]

• Mobile observers: highly discouraged; KiekR observers are dropped at ingest (subscriber OBSERVER_IGNORE_CLIENTS).
• "Why is my observer not listed?": check the map's Region → Observers popup.
• Native firmware: flasher steps are select device → select Repeater → flash.

These two observer commits were pushed to #30 after it merged, so they never reached main.

• mkdocs build --strict passes