docs: add UDS (Unlearning Depth Score) to evaluation metrics links

[gnueaj]

Adds a link to Unlearning Depth Score (UDS), a white-box metric that measures
unlearning depth via two-stage activation patching on internal hidden states,
evaluated on TOFU forget10 using open-unlearning's Llama-3.2-1B-Instruct
checkpoints (150 unlearned models, 8 methods).

Project Page: https://gnueaj.github.io/unlearning-depth-score/
Paper: https://arxiv.org/abs/2605.24614
Code: https://github.com/gnueaj/unlearning-depth-score

It's the first mechanistic metric for unlearning depth, and we thought it'd be
a nice addition to this list. Thank you for maintaining such a great resource
for the unlearning community 😊

Best,
Jaeung