docs: Update model compatibility documentation with missing backends (#8889)

* docs: Update model compatibility documentation with missing backends

Added the following backends to README.md and compatibility-table.md:
- vllm-omni: Multimodal vLLM with vision and audio support
- nemo: NVIDIA NeMo framework for speech models
- outetts: OuteTTS with voice cloning capabilities
- faster-qwen3-tts: Faster Qwen3 TTS implementation
- qwen-asr: Qwen automatic speech recognition
- voxcpm: VoxCPM speech understanding model
- whisperx: Enhanced Whisper with word-level transcription

These backends exist in the codebase (backend/index.yaml) but were missing
from the documentation. This update ensures accurate reflection of currently
supported backends in LocalAI.

* Apply suggestion from @mudler

Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

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Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@example.com>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>