docs(linkr-kvm): add Baidu Pan mirror download link to firmware update guide

[tangzz-radxa]

Summary

Add a Baidu Pan mirror download link to the Linkr KVM firmware update guide, to make it easier for mainland China users to grab the firmware when GitHub access is slow or limited.

Changes

• docs/linkr/linkr-kvm/firmware-update.mdx (CN)
  • New ### 国内镜像（百度网盘） subsection under ## 下载固件
  • Link: https://pan.baidu.com/s/1AZgLUDaU8hJKhZjqTT_sMQ?pwd=1234
  • Extraction code: 1234
  • Added :::caution 校验与安全 block advising filename/SHA256 verification and GitHub Releases as the source of truth
  • Updated ## 相关链接 with the mirror entry
• i18n/en/docusaurus-plugin-content-docs/current/linkr/linkr-kvm/firmware-update.mdx (EN)
  • Parallel ### China Mirror (Baidu Pan) subsection under ## Download the Firmware
  • Same link + extraction code, framed for China-specific GitHub access issues
  • Updated ## Related Links with the mirror entry

Context

• Source: internal request in 售后技术支持 (Feishu group oc_84f6a3a33b421fd57430899963d25d9d)
• Requested by: 吴闽龙
• Base branch: radxa-docs/docs upstream main
• Branch: docs/linkr-baidu-pan-firmware (forked from upstream-docs/main)

Notes

• The mirror is presented as a complement to the official GitHub Releases; GitHub Releases remains the source of truth and is referenced in the caution block.
• Only the firmware update guide is touched — no other docs are changed.
• Both CN and EN versions are updated together to keep them in sync.