[skills] model doc

[stevhliu]

make model addition easier by asking your agent to complete the model_doc/<model>.md file

1. transformers add-new-model-like creates a template with <PLACEHOLDER> tokens
2. trigger skill with something like "finish the docs for this new model"
3. the /model-doc skill uses the existing /hf-papers, /hf-models, and /hf-collections skills to look up the model to add relevant links to the model on the Hub and summarize what's new and cool about the model
4. any remaining <PLACEHOLDER> tokens are manually completed

see #45612 for example outputs for Qwen3.5 and Perception Encoder

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.