Add server credentialId docs (#669)

* custom credential authentication

* remove mdc: from non-cursor rule files

Author: AdiSai

fern/calls/assistant-based-warm-transfer.mdx

@@ -323,6 +323,6 @@ Configure your transfer assistant to:
 ## Next steps
 
 Now that you've configured assistant-based warm transfers:
-- **[Call forwarding](mdc:docs/call-forwarding):** Learn about other transfer modes and options
-- **[Assistant configuration](mdc:docs/assistants):** Configure assistant models and prompts
-- **[Custom tools](mdc:docs/tools/custom-tools):** Add custom tools to your assistants
+- **[Call forwarding](../call-forwarding):** Learn about other transfer modes and options
+- **[Assistant configuration](../assistants):** Configure assistant models and prompts
+- **[Custom tools](../tools/custom-tools):** Add custom tools to your assistants

fern/customization/custom-transcriber.mdx

@@ -408,9 +408,9 @@ You'll learn how to:
         "transcriber": {
           "provider": "custom-transcriber",
           "server": {
-            "url": "wss://your-server.ngrok.io/api/custom-transcriber"
-          },
-          "secret": "your_optional_secret_value"
+            "url": "wss://your-server.ngrok.io/api/custom-transcriber",
+            "credentialId": "cred_transcriber_auth_123"
+          }
         },
         "firstMessage": "Hello! I am using a custom transcriber with Deepgram."
       },
@@ -438,8 +438,8 @@ You'll learn how to:
 
 - **Streaming support requirement:**  
   The custom transcriber must support streaming. Vapi sends continuous audio data over the WebSocket, and your server must handle this stream in real time.
-- **Secret header:**  
-  The custom transcriber configuration accepts an optional field called **`secret`**. When set, Vapi will send this value with every request as an HTTP header named `x-vapi-secret`. This can also be configured via a headers field.
+- **Authentication:**  
+  For secure transcriber endpoints, use **Custom Credentials** with `credentialId`. Create [Custom Credentials](../server-url/server-authentication) in the dashboard to manage Bearer Token, OAuth 2.0, or HMAC authentication. For backward compatibility, the legacy `secret` field is still supported and sends the value as an `x-vapi-secret` HTTP header.
 - **Buffering:**  
   The solution buffers PCM audio and performs simple validation (e.g. ensuring stereo PCM data length is a multiple of 4). If the audio data is malformed, it is trimmed to a valid length.
 - **Channel detection:**  

fern/customization/custom-tts.mdx

@@ -53,30 +53,34 @@ VAPI's custom TTS system operates through a webhook pattern:
 
 ## Authentication setup
 
-VAPI needs secure communication with your TTS endpoint. Choose from these authentication options:
+VAPI needs secure communication with your TTS endpoint. Use **Custom Credentials** for authentication:
 
-### Secret header authentication
+### Using Custom Credentials (Recommended)
 
-The most common approach uses a secret token in the `X-VAPI-SECRET` header:
+Create authentication credentials in the dashboard and reference them by ID:
 
 <CodeBlocks>
-```json title="Assistant Configuration"
+```json title="Assistant Configuration with Custom Credentials"
 {
   "voice": {
     "provider": "custom-voice",
     "server": {
       "url": "https://your-tts-api.com/synthesize",
-      "secret": "your-secret-token-here",
+      "credentialId": "cred_tts_auth_123",
       "timeoutSeconds": 30
     }
   }
 }
 ```
 </CodeBlocks>
 
-### Enhanced authentication with custom headers
+<Note>
+Create [Custom Credentials](../server-url/server-authentication) in the Vapi dashboard for better security and credential management.
+</Note>
+
+### Legacy Authentication Methods
 
-Add extra headers for API versioning or enhanced security:
+For backward compatibility, you can still use inline authentication:
 
 <CodeBlocks>
 ```json title="Assistant Configuration with Custom Headers"