Wrote preliminary page on micro-ROS diagnostics.

Signed-off-by: Ralph Lange <ralph.lange@de.bosch.com>

Author: ralph-lange

_docs/concepts/client_library/diagnostics/diagnostics_architecture.png

@@ -3,6 +3,17 @@ title: Diagnostics
 permalink: /docs/concepts/client_library/diagnostics/
 ---
 
-**THIS PAGE IS UNDER CONSTRUCTION**
+Although diagnostics is not part of the ROS 2 Client Library packages (i.e., rclcpp, rclpy), it definitely can be counted to the extended client library as it provides very generic and application-independent functionalities.
+
+That's why the micro-ROS Client Library also comes with basic diagnostics functionalities. These are fully compatible with ROS 2 diagnostics, but comprise three features only:
+
+* Diagnostic message types (optimized for Micro-XRCE-DDS - no dynamic arrays)
+* Updater mechanisms for rclc
+* Selected basic diagnostic monitors
+
+The micro-ROS diagnostics packages do not provide any aggregation mechanisms as we assume that such aggregation takes place on a microprocessor running standard ROS 2. Hence, we assume the following typical architecture:
+
+<img src="diagnostics_architecture.png" style="display:block; width:60%; margin-left:auto; margin-right:auto;"/>
+
+For more information, please see [https://github.com/micro-ROS/micro_ros_diagnostics/](https://github.com/micro-ROS/micro_ros_diagnostics/).
 
-[https://github.com/micro-ROS/micro_ros_diagnostics](https://github.com/micro-ROS/micro_ros_diagnostics)