docs(orion): remove 飞牛 fnOS example download links

[tangzz-radxa]

Summary

• Remove the 飞牛 fnOS (FnNAS) example download links from the docs merged in docs(orion/o6): add 飞牛 fnOS install tutorial and download entry #1915.
• Keep only the "go to the FnNAS ARM download page and pick the UEFI ARM Installation Image ISO (armsr) yourself" instruction, in both Chinese and English.
• Touch the same 6 files as docs(orion/o6): add 飞牛 fnOS install tutorial and download entry #1915 (4 tutorial + 2 download page, CN/EN paired).

Why

Per follow-up feedback from the requester after #1915 was merged:

• The example direct download URL (https://iso.liveupdate.fnnas.com/.../fnos_Mainland-PE_arm_1.1.31_armsr_1366.iso) was pinned to a specific version (1.1.31 / 1366) that will go stale as FnNAS iterates the public beta.
• Customers should pull the image from the upstream FnNAS ARM download page so they always get the current build.
• Radxa docs should not re-host / re-link the FnNAS image CDN path; pointing to the official download page avoids drift from FnNAS's own distribution policy.

What Changed

Removed:

• The "当前推荐下载链接（示例）" / "Reference download link (example)" table (filename + version + direct URL) from the two fnos.md files (Orion O6 + Orion O6N, CN/EN).
• The "适用于此芯 P1 的镜像（UEFI ARM 安装镜像 ISO）" / "Image for CIX P1 (UEFI ARM Installation Image ISO)" direct-link table from docs/orion/download.md (CN/EN).

Replaced with (CN example):

适用于此芯 P1 的镜像请前往 飞牛 fnOS ARM 镜像下载页 的设备支持列表中查找 UEFI ARM 安装镜像 ISO 类别（文件标签 armsr）后自行下载。

镜像版本号与构建号会随飞牛 fnOS ARM 公测版本迭代而变化，请以上述下载页发布的最新版本为准。

The English equivalent is mirrored 1:1.

The MD5 verification tip in each tutorial is kept (it now refers to "the page you downloaded from" generically, no pinned URL).

Verification

• python3 scripts/github_pr_guard.py check --repo-path . --base radxa-docs-upstream/main --max-scopes 4 --fetch → ok: true, bilingual clean, 6 files, 1 commit, 3 scopes (docs/orion/download.md, docs/orion/o6, docs/orion/o6n).
• grep -rn 'iso.liveupdate\|fnos_Mainland-PE' docs/orion/{o6,o6n}/other-os/fnos.md docs/orion/download.md i18n/.../.../.../.../fnos.md i18n/.../download.md → no matches.
• Net diff: -28 lines, 0 specific file/version left in any docs page.