doc: add baseten provider

packages/web/src/content/docs/providers.mdx

@@ -229,6 +229,42 @@ Or if you already have an API key, you can select **Manually enter API Key** and
 
 ---
 
+### Baseten
+
+1. Head over to the [Baseten](https://app.baseten.co/), create an account, and generate an API key.
+
+2. Run `opencode auth login` and select **Baseten**.
+
+   ```bash
+   $ opencode auth login
+
+   ┌  Add credential
+   │
+   ◆  Select provider
+   │  ● Baseten
+   │  ...
+   └
+   ```
+
+3. Enter your Baseten API key.
+
+   ```bash
+   $ opencode auth login
+
+   ┌  Add credential
+   │
+   ◇  Select provider
+   │  Baseten
+   │
+   ◇  Enter your API key
+   │  _
+   └
+   ```
+
+4. Run the `/models` command to select a model.
+
+---
+
 ### Cerebras
 
 1. Head over to the [Cerebras console](https://inference.cerebras.ai/), create an account, and generate an API key.