Clarify Intel XPU support gaps and unsupported model scope in XPU model matrix docs

[Copilot]

The issue asked for a clear view of what Intel XPU still lacks and which models are not supported. This PR adds that missing summary directly to the Intel XPU hardware support page so users can quickly determine current capability boundaries.

• Documentation: Intel XPU feature gaps

  • Added a new Current Gaps on Intel XPU section in docs/models/hardware_supported_models/xpu.md.
  • Captures key runtime limitations currently relevant to users:
    • CUDA graph mode not available on Intel XPU
    • Flash Attention with float32 falls back to Triton
    • bfloat16 blocked on Arc A770 due to known accuracy issues
    • Additional XPU graph capture constraints in multi-GPU communication scenarios

• Documentation: Intel GPU quantization gaps

  • Added an explicit unsupported quantization list for Intel GPU, sourced from the quantization hardware matrix:
    • Marlin (GPTQ/AWQ/FP8/FP4)
    • INT8 (W8A8)
    • FP8 (W8A8)
    • bitsandbytes
    • DeepSpeedFP
    • GGUF
  • Added a direct relative link to the quantization matrix for traceability.

• Documentation: unsupported model interpretation

  • Added a Model Support Scope section clarifying that the page is a validated allowlist, not an exhaustive unsupported-model list.
  • Defined practical criteria for treating a model as unsupported / not yet validated on Intel XPU.

Example snippet from the new guidance:

For Intel XPU, treat a model as **not supported / not yet validated** when any of the following is true:

- The model architecture or checkpoint is **not listed** in the validated tables below.
- The model depends on a quantization method listed above as unsupported on Intel GPU.
- The model only works with unsupported XPU feature combinations.

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