docs: refine FEP template Packaging and Test Plan sections

[Al2O3Al2O3]

• Packaging: focus on build/packaging only (not install); add supported vendors field; center on whether the feature can ship as a .whl, with build steps or a prebuilt wheel when yes, and build-from-source when no
• Test Plan: drop image/install/setup steps; keep test commands and expected results, scoped to the new features in this release

[CLAassistant]

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[wbavon]

LGTM — 方向正确，关注点分离后模板更清晰。

几点小建议供后续迭代参考（不阻塞合并）：

1. Test Plan 可考虑加一行可选的 Prerequisites/Environment 字段，避免验证者不知如何搭建环境
2. Packaging 的决策树可以稍微泛化，覆盖 Docker/system package 等非-wheel 场景
3. 注释中 python -m build 和 pip wheel 二选一即可，减少新手困惑

注意 CLA 状态需要解决后才能合并。