docs: refresh for native kernel repo type on the Hub

[gary149]

Tried to keep it minimal but still need a quick check @danieldk @drbh - feel free to make changes or add more details directly :) cc @julien-c too btw

Summary

Minimal-touch refresh of the kernels docs to reflect the native Kernel Hub support that shipped at PyTorch Conference Europe (Apr 2026), and corrects a few outdated references.

• Discovery URLs: huggingface.co/models?other=kernels → huggingface.co/kernels (in index.md and integrating-kernels.md).
• Landing-page thumbnail (index.md): replace the GitHub attachment with light/dark variants stored in huggingface/documentation-images (kernels/kernels-thumbnail-{light,dark}.png), wrapped in a link to huggingface.co/kernels.
• index.md framing: one-sentence note that kernels are a first-class Hub repo type with hardware/version pages.
• migration.md: new ## 0.14 section covering the kernel-only loader (kernels/src/kernels/utils.py hardcodes repo_type="kernel" after b26b7f6 "remove backward compat of model repo types"), kernel-creation access flow, and steps to migrate an existing model-type repo.
• builder/build.md: short prerequisite note above build-and-upload pointing to huggingface.co/settings/account ("Request Kernels Creation").
• kernel-requirements.md: new "Repository type" and "Trusted publishers" sections. The latter documents the actual loader behavior — non-trusted publishers raise ValueError unless the caller passes trust_remote_code=True — and notes the Hub-side trustedKernelPublisher flag.

Did not touch: _toctree.yml, builder/security.md (artifact signing isn't shipped yet), the (./why_kernels.md) link style or other pre-existing relative .md links.

Test plan

• doc-builder build kernels docs/source/ --html builds clean
• Previewed locally via moon-landing (/docs/kernels/main/en/{index,migration,builder/build,kernel-requirements,integrating-kernels})
• Light/dark image swap verified by toggling system theme
• CI doc build green

[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.

[danieldk]

Looks awesome, thanks!