Fix invalid examples in QwenVL model docstrings and add Qwen3VL example
Author: Xqle
Pull request: yes

# What does this PR do?
This PR fixes the non-functional examples for Qwen2-VL and Qwen2.5-VL models, and adds a runnable example for Qwen3VL.
<!--
Congratulations! You've made it this far! You're not quite done yet though.

Once merged, your PR is going to appear in the release notes with the title you set, so make sure it's a great title that fully reflects the extent of your awesome contribution.

Then, please replace this with a description of the change and which issue is fixed (if applicable). Please also include relevant motivation and context. List any dependencies (if any) that are required for this change.

Once you're done, someone will review your PR shortly (see the section "Who can review?" below to tag some potential reviewers). They may suggest changes to make the code even better. If no one reviewed your PR after a week has passed, don't hesitate to post a new comment @-mentioning the same persons---sometimes notifications get lost.
-->

<!-- Remove if not applicable -->

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@zucchini-nlp, @Rocketknight1

<!-- Your PR will be replied to more quickly if you can figure out the right person to tag with @

 If you know how to use git blame, that is the easiest way, otherwise, here is a rough guide of **who to tag**.
 Please tag fewer than 3 people.

Models:

- text models: @ArthurZucker @Cyrilvallez
- vision models: @yonigozlan @molbap
- audio models: @eustlb @ebezzam @vasqu
- multimodal models: @zucchini-nlp
- graph models: @clefourrier

Library:

- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- continuous batching: @remi-or @ArthurZucker @McPatate
- pipelines: @Rocketknight1
- tokenizers: @ArthurZucker and @itazap
- trainer: @SunMarc
- attention: @vasqu @ArthurZucker @CyrilVallez
- model loading (from pretrained, etc): @CyrilVallez
- distributed: @3outeille @ArthurZucker
- CIs: @ydshieh

Integrations:

- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization: @SunMarc @MekkCyber
- kernels: @MekkCyber @drbh
- peft: @BenjaminBossan @githubnemo

Devices/Backends:

- AMD ROCm: @ivarflakstad
- Intel XPU: @IlyasMoutawwakil
- Ascend NPU: @ivarflakstad 

Documentation: @stevhliu

Research projects are not maintained and should be taken as is.

 -->


Comments:
@zucchini-nlp, @Rocketknight1
Hi , I noticed that the `build / build_pr_documentation` check has been pending for a while and all other checks have passed.
Do I need to do anything to make this check pass?

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: qwen2_5_vl, qwen2_vl, qwen3_vl

Yes, sorry, this needed to be approved by a maintainer! After the job is finished you should get a link to the built docs - please check that the examples look right there and then ping me for final review.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_41812). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@Rocketknight1 
I‘ve checked the updated examples in the PR docs ([Qwen2VL](https://moon-ci-docs.huggingface.co/docs/transformers/pr_41812/en/model_doc/qwen2_vl#transformers.Qwen2VLForConditionalGeneration), [Qwen2.5VL](https://moon-ci-docs.huggingface.co/docs/transformers/pr_41812/en/model_doc/qwen2_5_vl#transformers.Qwen2_5_VLForConditionalGeneration), [Qwen3VL](https://moon-ci-docs.huggingface.co/docs/transformers/pr_41812/en/model_doc/qwen3_vl#transformers.Qwen3VLForConditionalGeneration)) and everything LGTM.

The fixes also LGTM, so happy to merge, in that case! Thank you for the PR.