[docs] Update index

[stevhliu]

Updates index.md since we've moved a bit away from the older layout of tutorials/guides/conceptual guides/reference.

[HuggingFaceDocBuilderDev]

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[sayakpaul]

Nice but curious to know why moved away from the previous layout as it looked quite complete to me.

[stevhliu]

We don't have a dedicated sections for tutorials/guides/conceptual guides/reference in the toctree, so I think it makes more sense to remove those buttons to be more aligned. I want users to think about the docs more in terms of topics they're interested in rather than whether they're looking for a tutorial, guide, etc.

Hope that makes sense! 🤗