Fix remaining broken pretrain_distill docs links

[minutechreview]

Follow-up to #934, picking up the instances I listed there but deliberately left out of that PR.

#934 fixed the two occurrences the issue named (README + docs landing page). The same dead base URL was still present in three more places, all now 404:

file	before	after
src/lightly_train/_cli.py:57	pretrain_distill.html	pretrain_distill/index.html
src/lightly_train/_commands/train.py:88	pretrain_distill.html	pretrain_distill/index.html
examples/notebooks/distillation.ipynb:384	pretrain_distill.html	pretrain_distill/index.html
examples/notebooks/distillation.ipynb:90	pretrain_distill.html#train-data	pretrain_distill/index.html#pretrain-data

The fourth one

In #934 I flagged the notebook's data-guide link as needing a decision, because its #train-data anchor doesn't exist. I've since found the intended target: the Data section in docs/source/pretrain_distill/index.md is anchored (pretrain-data)= (line 120), not train-data. So that link now points at #pretrain-data.

If that isn't the section you meant it to reach, say so and I'll change it — but it's the only Data anchor in that page.

Verification

• pretrain_distill.html → 404; pretrain_distill/index.html → 200
• id="pretrain-data" is present on the rendered page; id="train-data" is not
• pretrain_distill/index.html is already the form used elsewhere in these same files (e.g. the #resume-training links at _cli.py:119/133), so this is consistent rather than a new convention
• distillation.ipynb still parses as valid JSON (21 cells)
• _cli.py and _commands/train.py both compile

After this, grep -rn "pretrain_distill\.html" returns nothing across the repo.

[mrpositron]

/review

[liopeer]

Great, thanks!