docs: add manual voice ID entry instructions to legacy migration guide (#913)

* docs: add manual voice ID entry instructions to legacy migration guide

Add note explaining how users can manually enter voice IDs for voices
not listed in the Vapi Dashboard dropdown by using the "Add Voice ID
Manually" option.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add FAQ about ElevenLabs model compatibility

Add new FAQ entry clarifying that replacement voices work with any
ElevenLabs models out of the box.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Vapi Tasker <tasker@vapi.ai>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: 3 people

fern/providers/voice/vapi-voices/legacy-migration.mdx

@@ -66,6 +66,8 @@ We've selected replacement voices from ElevenLabs using their voice matching too
 | Lily | Tanya - `7sAIIIXrQKek5xiLc1R6` | Melissa - `JH3fX8OSjg6sNdEtPjxr` | Ariel - `TPbSfDVirzpiSkim8gMw` |
 | Kylie | Chanelle - `t5ztDJA7pj9EyW9QIcJ2` | Aryannah - `vOJuCTA36jkG6q1OskrK` | Riley - `kXsOSDWolD7e9l1Z0sbH` |
 
+For voices that are not in the list of voices on the Vapi Dashboard, users will need to check the "Add Voice ID Manually" option and paste the Voice ID in the text box.
+
 ## FAQs
 
 <AccordionGroup>
@@ -103,4 +105,8 @@ We've selected replacement voices from ElevenLabs using their voice matching too
   <Accordion title="Are the staying voices (Elliot, Savannah, Leo, etc.) affected?">
     No. Elliot, Savannah, Leo, Zoe, Mia, Jess, Zac, Dan, Leah, Tara, and Rohan are continuing and are not being retired. If you're using one of these voices, no action is required.
   </Accordion>
+
+  <Accordion title="Are these voices compatible with specific ElevenLabs models?">
+    Replacement voices should work out of the box with any of the ElevenLabs models.
+  </Accordion>
 </AccordionGroup>