Start promoting Xet more accross the doc (not just in a single doc page)

[julien-c]

• move Storage limits and Storage backends up in the ToC
• small presentation (including a visual) of Xet on the repos index page: https://huggingface.co/docs/hub/repositories
• and also a mention on the main index page

[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.

[jsulz]

Nice! Love getting Xet in more places in the docs. Definitely see a few more places to update. I'll get going on them.

[julien-c]

Thanks for the reviews, merging!