docs: update navigation for voice and transcriber fallback configurations

Renamed the "Voice fallback plan" to "Voice fallback configuration" and added a new entry for "Transcriber fallback configuration" in the documentation navigation. This enhances clarity and accessibility for users seeking information on fallback configurations.

Author: vtkovapi

fern/docs.yml

@@ -160,8 +160,10 @@ navigation:
                 path: customization/speech-configuration.mdx
               - page: Voice pipeline configuration
                 path: customization/voice-pipeline-configuration.mdx
-              - page: Voice fallback plan
+              - page: Voice fallback configuration
                 path: voice-fallback-plan.mdx
+              - page: Transcriber fallback configuration
+                path: customization/transcriber-fallback-plan.mdx
               - page: OpenAI realtime speech-to-speech
                 path: openai-realtime.mdx
               - page: Provider keys