docs(orion/o6): add 飞牛 fnOS install tutorial and download entry

[tangzz-radxa]

Summary

• Add 飞牛 fnOS (FnNAS) install tutorial under Orion O6 "Other OS" section.
• Update Orion download page (docs/orion/download.md) to add the FnNAS image entry alongside Radxa OS / Android / OpenHarmony / Fedora / Debian 13.
• Mirror the Chinese changes into the English docs (i18n/en/docusaurus-plugin-content-docs/current/...).

Why

A support request was opened to add FnNAS / 飞牛 fnOS to the official docs for Radxa Orion O6, since the FnNAS ARM public beta now explicitly lists CIX P1 (the SoC used by Orion O6) as supported under the "UEFI ARM Installation Image ISO" category on https://www.fnnas.com/download-arm. Customers on the "Other OS" page need a single-link guide that points to the upstream FnNAS install tutorial plus the supported CIX P1 image entry.

The tutorial is intentionally short and redirects to the FnNAS upstream tutorial for the actual install steps, keeping the Radxa docs page focused on:

• verifying scope and current public-beta status
• pointing to the supported image category
• calling out risks, BIOS prerequisites, and where to get upstream help

What Changed

• docs/orion/o6/other-os/fnos.md (new): installation tutorial for FnNAS fnOS ARM public beta on Orion O6.
• i18n/en/docusaurus-plugin-content-docs/current/orion/o6/other-os/fnos.md (new): English mirror.
• docs/orion/download.md: new "FnNAS fnOS" section under System Images, plus updated quick-navigation text.
• i18n/en/docusaurus-plugin-content-docs/current/orion/download.md: English mirror.

The page already lists 16 Radxa boards under the FnNAS ARM device support list (Cubie A5E, E20C/E24C/E25/E52C/E54C, ROCK 4D/5A/5B/5B+/5C/5C Lite/5T/5 ITX/2A/2F), confirming the existing Radxa–FnNAS collaboration.

Scope / Status Notes

• Scope is intentionally limited to Orion O6 (per the original ask that referenced https://docs.radxa.com/orion/o6/other-os). O6N coverage is not included in this PR; can be added as a follow-up since the same CIX P1 ISO applies.
• FnNAS fnOS ARM is currently in public beta (1.1.31 / 1366 as of writing). The tutorial clearly labels it as community-validated, not an officially supported Radxa distribution, and points users to the FnNAS public-beta disclaimer.
• The image entry on the download page references the current 1.1.31 / 1366 build and notes that the version / build may change; readers are directed to the FnNAS ARM download page for the latest.

Verification

• python3 scripts/github_pr_guard.py check --base radxa-docs-upstream/main --max-scopes 3 → ok: true, bilingual pair clean, 4 files changed, 2 scopes.
• Filenames, model names, BIOS menu paths (Device Manager → Platform Configuration → Compliance Configuration → Enable ACPI SCMI), and the upstream FnNAS URLs were cross-checked against:
  • https://www.fnnas.com/download-arm
  • https://help.fnnas.com/articles/v1/start/install-os
  • https://help.fnnas.com/articles/v1/contact/disclaimers.md
  • https://help.fnnas.com/articles/fnosV1/start/install-os-fail.md
  • https://help.fnnas.com/articles/fnosV1/volume/create.md
  • https://help.fnnas.com/articles/v1/access/how-access
  • https://help.fnnas.com/articles/fnosV1/start/install-app.md
• The BIOS reference matches the existing BIOS Update page in this docs tree.

Follow-Up Suggestions

• If a Radxa customer validates FnNAS running stably on Orion O6 / O6N end-to-end, consider promoting the entry from "FnNAS fnOS" to a separate sibling row in the System Images section (similar to how Debian 13 is presented).
• Consider adding a mirroring entry under docs/orion/o6n/other-os/fnos.md once the customer confirms O6 first-boot success; the same CIX P1 ISO applies.