docs(contest): add Agora & Beken conversational AI dev kit to to-be-ported boards

[tanghao-xiaomi]

What changed

Added a new entry to the 待适配开发板 (to-be-ported boards) section of supported_hardware.md:

6、对话式 AI 开发套件 — 声网 & 博通集成 (Agora & Beken Conversational AI Dev Kit)

Sourced from the official contest wiki's to-be-ported hardware table. Fields follow the same format as the existing entries (chip features, use cases, device intro, technical materials, upstream target repo).

• Product image downloaded and added at images/agora_beken_convai.png.
• Technical materials PDF (声网开发套件使用说明.pdf) added under attachment/ and linked from the entry, matching how LogicPi A1 / BES2800BP reference their local materials.
• Note on upstream repo: the kit's chip side is from Beken, so the upstream target repo is set to vendor_beken with a "以仓库实际为准" (subject to actual repo) qualifier — please correct if the actual vendor attribution differs.

Testing

Documentation-only change; no build required.