[ZeroGPU] Tiers section + ZeroGPU in docs index

docs/hub/spaces-zerogpu.md

@@ -13,7 +13,7 @@ Unlike traditional single-GPU allocations, ZeroGPU's efficient system lowers bar
 
 - **Using existing ZeroGPU Spaces**
   - ZeroGPU Spaces are available to use for free to all users. (Visit [the curated list](https://huggingface.co/spaces/enzostvs/zero-gpu-spaces)).
-  - [PRO users](https://huggingface.co/subscribe/pro) get x5 more daily usage quota and highest priority in GPU queues when using any ZeroGPU Spaces.
+  - [PRO users](https://huggingface.co/subscribe/pro) get x7 more daily usage quota and highest priority in GPU queues when using any ZeroGPU Spaces.
 - **Hosting your own ZeroGPU Spaces**
   - Personal accounts: [Subscribe to PRO](https://huggingface.co/settings/billing/subscription) to access ZeroGPU in the hardware options when creating a new Gradio SDK Space.
   - Organizations: [Subscribe to the Enterprise Hub](https://huggingface.co/enterprise) to enable ZeroGPU Spaces for all organization members.
@@ -119,6 +119,16 @@ ZeroGPU does not support `torch.compile`, but you can use PyTorch **ahead-of-tim
 
 Check out this [blogpost](https://huggingface.co/blog/zerogpu-aoti) for a complete guide on ahead-of-time compilation on ZeroGPU.
 
+## Usage Limitations
+
+GPU usage is subject to **daily** quotas, per account tier:
+
+- **Unlogged**: 2 minutes
+- **Free account**: 3.5 minutes
+- **Pro account**: 25 minutes (7x more than free account)
+
+Higher quota also means higher prioirty in ZeroGPU queues
+
 ## Hosting Limitations
 
 - **Personal accounts ([PRO subscribers](https://huggingface.co/subscribe/pro))**: Maximum of 10 ZeroGPU Spaces.