docs: add Gandr to the Custom TTS section

[AALG123]

Adds Gandr to the Custom TTS section, next to the existing Gradium page, so people who arrive through Custom TTS find the same integration that already lives under Custom voices (#1136). Also carries the small wording pass from #1176, closed in favour of this one so there is a single Gandr docs PR open.

• One new page: fern/customization/custom-tts/gandr.mdx, in the same shape as the Gradium Custom TTS page: the endpoint choice (Gandr's hosted /v1/vapi endpoint, or the deployable adapter at github.com/Gandr-AI/gandr-vapi for teams that want the TTS hop on their own server), the assistant config, troubleshooting and related links
• Registered in fern/docs.yml under Custom TTS
• Existing Custom voices page: tighter first sentence and one sentence pointing at the self-hosted adapter (the docs: tighten the Gandr custom-voice page wording #1176 change)
• Cross-links the existing Custom voices page instead of duplicating its dashboard walkthrough
• No pricing, no provider page touched, zero Vapi engineering required

Disclosure: I work on Gandr.

[lightsage-app]

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Commit: ed009c2
Status: waiting for staging docs URL