Fix moved Wuerstchen example path in training docs

[latent-9]

What does this PR do?

The Wuerstchen text-to-image training example lives at examples/research_projects/wuerstchen/text_to_image/, but the training docs still reference the old top-level examples/wuerstchen/text_to_image path:

• docs/source/en/training/wuerstchen.md: the runnable cd examples/wuerstchen/text_to_image fails with No such file or directory (so the following pip install -r requirements.txt fails too), and 6 source/blob links 404.
• docs/source/en/training/overview.md and lora.md: the Wuerstchen links 404.

This updates all references (9 across the three files) to the current examples/research_projects/wuerstchen/text_to_image path. Verified the directory exists there (train_text_to_image_prior.py, train_text_to_image_lora_prior.py, modeling_efficient_net_encoder.py, requirements.txt).

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

[github-actions]

Hi @latent-9, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.