docs: Move MQTT_DIAGNOSTICS.rst to guides/mqtt_diagnostics.rst

- Rename file to lowercase following convention
- Update documentation index to reference new location

Author: eman

docs/MQTT_DIAGNOSTICS.rst docs/guides/mqtt_diagnostics.rstdocs/MQTT_DIAGNOSTICS.rst renamed to docs/guides/mqtt_diagnostics.rst

@@ -127,6 +127,7 @@ Documentation Index
    guides/event_system
    guides/command_queue
    guides/auto_recovery
+   guides/mqtt_diagnostics
    guides/advanced_features_explained
 
 .. toctree::