Add ROS2 and Diffusion Policy guides with link standardizations

This PR adds several new wiki entries and improves project-wide consistency:
- New page: ROS2 Custom Packages guide
- New page: Diffusion Models and Diffusion Policy introduction
- New page: GRPO for Diffusion Policies
- New page: ROS2 Humble IPC Recorder tool
- Renamed `nlp_for_robotics.md` to `nlp-for-robotics.md` for kebab-case compliance.
- Standardized internal links in modified files to use trailing slashes and remove `.md` extensions.
- Updated `_data/navigation.yml` and `wiki/machine-learning/index.md` for new content and consistent naming.

Author: Purenothingness24

_data/navigation.yml

@@ -100,6 +100,8 @@ wiki:
         url: /wiki/common-platforms/ros2-navigation-for-clearpath-husky/
       - title: Hello Robot Stretch RE1
         url: /wiki/common-platforms/hello-robot
+      - title: Building ROS2 Custom Packages
+        url: /wiki/common-platforms/ros/ros2-custom-package/
   - title: Sensing
     url: /wiki/sensing/
     children:
@@ -177,14 +179,18 @@ wiki:
         url: /wiki/machine-learning/python-libraries-for-reinforcement-learning/
       - title: Reinforcement Learning
         url: /wiki/machine-learning/intro-to-rl/
+      - title: Diffusion Models and Diffusion Policy
+        url: /wiki/machine-learning/intro-to-diffusion/
+      - title: GRPO for Diffusion Policies
+        url: /wiki/machine-learning/grpo-diffusion-policies/
       - title: YOLO Integration with ROS and Running with CUDA GPU
         url: /wiki/sensing/ros-yolo-gpu/
       - title: YOLOv5 Training and Deployment on NVIDIA Jetson Platforms
         url: /wiki/machine-learning/yolov5-tensorrt/
       - title: Mediapipe - Live ML anywhere
         url: /wiki/machine-learning/mediapipe-live-ml-anywhere/
       - title: NLP for robotics
-        url: /wiki/machine-learning/nlp_for_robotics/
+        url: /wiki/machine-learning/nlp-for-robotics/
   - title: State Estimation
     url: /wiki/state-estimation/
     children:
@@ -348,6 +354,8 @@ wiki:
         url: /wiki/tools/code-editors-Introduction-to-vs-code-and-vim/
       - title: Qtcreator UI development with ROS
         url: /wiki/tools/Qtcreator-ros/
+      - title: ROS2 Humble Intra-Process Communication Recorder
+        url: /wiki/tools/ros2-humble-ipc-recorder/
   - title: Datasets
     url: /wiki/datasets/
     children:

wiki/common-platforms/ros/ros2-custom-package.md

@@ -17,10 +17,16 @@ This section demonstrates how machine learning enhances robotic systems by enabl
 - **[Introduction to Reinforcement Learning](/wiki/machine-learning/intro-to-rl/)**
   Covers reinforcement learning concepts and Bellman equations. Discusses methods like dynamic programming, Monte Carlo, and temporal difference learning, with an emphasis on robotic applications.
 
+- **[Introduction to Diffusion Models and Diffusion Policy](/wiki/machine-learning/intro-to-diffusion/)**
+  Comprehensive introduction to diffusion models and their application in robotics through diffusion policies. Covers ODE and SDE formulations, their practical implications, and how diffusion policies enable multi-modal action learning for complex robotic tasks.
+
+- **[GRPO for Diffusion Policies in Robotics](/wiki/machine-learning/grpo-diffusion-policies/)**
+  Introduces Group Relative Policy Optimization (GRPO) and its application to diffusion policies using SDE formulation for stochastic sampling. Covers GRPO's origins in LLMs, the mathematical framework, and practical implementation strategies for optimizing robot policies with reward-based learning.
+
 - **[Mediapipe: Real-Time ML for Robotics](/wiki/machine-learning/mediapipe-live-ml-anywhere/)**
   Introduces MediaPipe for live ML inference on various platforms, including Android, iOS, and IoT. Highlights body pose tracking, hand tracking, and object detection pipelines.
 
-- **[NLP for Robotics](/wiki/machine-learning/nlp_for_robotics/)**
+- **[NLP for Robotics](/wiki/machine-learning/nlp-for-robotics/)**
   Explores how natural language processing (NLP) enables robots to understand and respond to human language. Includes an overview of transformer models and HuggingFace library usage.
 
 - **[Python Libraries for Reinforcement Learning](/wiki/machine-learning/python-libraries-for-reinforcement-learning/)**