Add predoc information to voice-formatting-plan (#244)

Author: skeptrunedev

fern/assistants/voice-formatting-plan.mdx

@@ -1,3 +1,10 @@
+---
+title: Voice Formatting Plan
+subtitle: >-
+  Learn what voice formatting plans are and how to format voice input dynamically for clearer and more natural text-to-speech interactions.
+slug: assistants/voice-formatting-plan
+---
+
 ## What is Voice Input Formatted?
 
 When interacting with voice assistants, you might notice terms like `Voice Input Formatted` in call logs or system outputs. This article explains what this means, how it works, and why it's important for delivering clear and natural voice interactions.