feat: add Gladia.io STT provider documentation

[jqueguiner]

Summary

• Adds a new integration guide for Gladia.io as a Speech-to-Text provider (gladia-stt-integration.md)
• Updates the STT configuration overview page to list Gladia as a supported provider with a link to the guide
• Updates the environment variables page to include AUDIO_STT_GLADIA_API_KEY and gladia as an engine option

This documents the Gladia STT integration added in Open WebUI, which uses Gladia's pre-recorded transcription API (v2) with a 3-step async workflow (upload → transcribe → poll).

Test plan

• Verify the new Gladia integration guide renders correctly
• Verify links from stt-config.md to the Gladia guide work
• Verify env-variables.md correctly lists the Gladia section

[pr-validator-bot]

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