[docs] 新增 xllm：快速上手链到官方 NPU 启动文档

[yzeyu71]

概述

为 docs 仓库新增 xllm 项目入口。xllm 官方文档站已提供完整的昇腾 NPU 启动说明（启动 xllm），因此本仓只保留指向官方文档的链接页：不自写教程、不建 quick-start 看护，避免与上游文档双头维护。

变更内容

文件	说明
sources/xllm/index.rst	单文件外链页（RST 原生语法），与 sources/sglang/index.rst 同构：GitHub / 文档中心 / NPU 启动指南三条官方链接
index.rst	「高性能推理与服务」分区新增 xllm 卡片（置于 vLLM-Ascend 之后）+ toctree 条目（置于 vllm-ascend 之后）；已有内容零改动，仅两处纯新增

单文件结构不涉及 conf.py 改动：页面内容直接写在 index.rst 里，无需 include 双文件结构与对应的 exclude_patterns 排除。

参考

• 结构参考：sources/sglang/index.rst（官方文档外链型项目的既有惯例）
• 官方 NPU 文档：https://docs.xllm-ai.com/zh/getting_started/launch_xllm/

测试计划

• PR 预览确认 xllm 卡片渲染与侧边栏导航正常
• 确认外链页三条链接可达

[ascend-robot]

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[ascend-robot]

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@yzeyu71 , thanks for your pull request.

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yzeyu71, thanks for your pull request. All authors of the commits have signed the CLA. 👍

[github-actions]

📖 文档预览：https://ascend.github.io/docs/pr-preview/pr-168/sources/xllm/

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构建自 142bb2d27853f2e9170603dbe1236c5197e5b46d。
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