Reorganized client library section. (#281)

* Reorganized pages in client library section.

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

* Updated intro text on Client Library.

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

* Moved decision paper to a PDF file.

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

* Wrote preliminary page on micro-ROS diagnostics.

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

* Create basic feature list page.

Author: ralph-lange

_data/docs.yml

@@ -12,11 +12,11 @@
 
 - title: Client Library
   docs:
-   - concepts/client_library
-   - concepts/client_library/real-time_executor
-   - concepts/client_library/system_modes
-   - concepts/client_library/decision_paper
-   - concepts/client_library/rcutils_analysis
+   - concepts/client_library/introduction
+   - concepts/client_library/features
+   - concepts/client_library/execution_management
+   - concepts/client_library/lifecycle_and_system_modes
+   - concepts/client_library/diagnostics
 
 - title: Middleware
   docs:

_docs/concepts/client_library/decision_paper/allocations_Fast-RTPS.png

@@ -0,0 +1,18 @@
+---
+title: Diagnostics
+permalink: /docs/concepts/client_library/diagnostics/
+---
+
+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 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/).