suggestion: add PKU-Alignment/BeaverTails and R-Judge dataset into safety section

[yanan1116]

i would suggest to add the following datasets into safety section:

• classification dataset: https://huggingface.co/datasets/PKU-Alignment/BeaverTails
• preference dataset: https://huggingface.co/datasets/PKU-Alignment/PKU-SafeRLHF
• R-judge: https://rjudgebench.github.io/index.html#dataset