docs(devices): publish ILX MultiPad first case

[ericshang98]

Summary

• publish ILX MultiPad as the first complete public hardware case
• pin the upstream firmware commit and document the adapter integration boundary
• add build/backup/verification and conditional Module/FPC flash guidance
• add the case to the public SDK reader entry points and contract tests

Verification

• source-first export completed from open-source/nexting-devices
• public npm run check passed during sync
• website documentation contracts and homepage scroll checks passed in the private source checkout
• no original upstream leden.hex is advertised as a Nexting image