IRM and APO comparisons

[SvenKlaassen]

• Add IRM vs APO example
• Correct links to stable version in CATE and GATE examples

[PhilippBach]

Hi @SvenKlaassen

I had a look at this PR and just made some minor adjustments. One thing I added was the original score for the "ATTE" in the IRM. I think the initial presentation from Chernozhukov et al. (2018) might be helpful for users. Feel free to adjust the corresponding section as you like.

Thanks for adding the nice IRM vs APO example.

Simulations on doubleml-coverage for ATE using IRM ATE and APO ATE are currently running, see for example https://github.com/DoubleML/doubleml-coverage/actions/runs/13366074711/job/37324039663 . I will open a PR and link it to this PR once results are ready.