chore: remove the sponsor material for Doubao and Atlas Cloud

The about pages lose the "Sponsored by ByteDance Doubao" card, the
promotional links in the provider list and the thanks paragraph; the
"bring your own key" note stays. The quota toast no longer shows the
Volcengine sign-up paragraph, and its messages in the four languages no
longer mention a sponsor. The provider guides lose the "free tokens"
promo line. The logo and invite images used only by that material are
deleted. Doubao and Atlas Cloud remain supported providers.

docs/cn/ai-providers.md

@@ -13,8 +13,6 @@
 
 ### 豆包 (字节跳动火山引擎)
 
-> **免费 Token**：在 [火山引擎 ARK 平台](https://www.volcengine.com/activity/codingplan?ac=MMAP8JTTCAQ2&rc=Z9Z3LDTJ&utm_campaign=drawio&utm_content=drawio&utm_medium=devrel&utm_source=OWO&utm_term=drawio) 注册，即可获得所有模型 50 万免费 Token！
-
 ```bash
 DOUBAO_API_KEY=your_api_key
 AI_MODEL=doubao-seed-1-8-251215  # 或其他豆包模型

docs/en/ai-providers.md

@@ -13,8 +13,6 @@ This guide explains how to configure different AI model providers for next-ai-dr
 
 ### Doubao (ByteDance Volcengine)
 
-> **Free tokens**: Register on the [Volcengine ARK platform](https://www.volcengine.com/activity/codingplan?ac=MMAP8JTTCAQ2&rc=Z9Z3LDTJ&utm_campaign=drawio&utm_content=drawio&utm_medium=devrel&utm_source=OWO&utm_term=drawio) to get 500K free tokens for all models!
-
 ```bash
 DOUBAO_API_KEY=your_api_key
 AI_MODEL=doubao-seed-1-8-251215  # or other Doubao model

docs/ja/ai-providers.md

@@ -13,8 +13,6 @@
 
 ### Doubao (ByteDance Volcengine)
 
-> **無料トークン**: [Volcengine ARK プラットフォーム](https://www.volcengine.com/activity/codingplan?ac=MMAP8JTTCAQ2&rc=Z9Z3LDTJ&utm_campaign=drawio&utm_content=drawio&utm_medium=devrel&utm_source=OWO&utm_term=drawio)に登録すると、すべてのモデルで使える50万トークンが無料で入手できます！
-
 ```bash
 DOUBAO_API_KEY=your_api_key
 AI_MODEL=doubao-seed-1-8-251215  # または他の Doubao モデル