Enhance dataset robustness discussion and integrate joint position histograms

- Expanded the explanation on dataset robustness, highlighting the benefits of aggregating data from multiple setups and organizations.
- Emphasized how the LLT Franka Emika Robot reflects fewer workspace constraints compared to ITA, supporting the discussion on uniform distribution and diversity.
- Added a comprehensive figure caption to clarify the differences in joint position distributions across datasets.
- Connected the discussion to the WWL paradigm, showcasing its role in enhancing dataset reliability and diversity.