Fix ReadTheDocs build: move LTX-2 docs to proper location and add to nav

Co-authored-by: hsliuustc0106 <222337142+hsliuustc0106@users.noreply.github.com>

docs/user_guide/examples/offline_inference/ltx2.md

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+# LTX-2 Text-To-Video
+
+Source <https://github.com/vllm-project/vllm-omni/tree/main/examples/offline_inference/ltx2>.
+
+The `Lightricks/LTX-2` pipeline generates high-quality videos from text prompts using a 19B parameter DiT architecture. It supports up to 4K video generation with synchronized audio.
+
+## Local CLI Usage
+
+```bash
+python text_to_video.py \
+  --prompt "A panda riding a bicycle through a forest, cinematic lighting" \
+  --height 512 \
+  --width 768 \
+  --num_frames 121 \
+  --num_inference_steps 40 \
+  --guidance_scale 4.0 \
+  --output ltx2_output.mp4
+```
+
+Key arguments:
+
+- `--prompt`: Text description of the video to generate (string).
+- `--height/--width`: Output resolution (defaults 512x768). Dimensions must be divisible by 32.
+- `--num_frames`: Number of frames (should be 8*n+1, e.g., 25, 81, 121). Default is 121.
+- `--guidance_scale`: Classifier-free guidance scale (default: 4.0). Range 3.0-5.0 recommended.
+- `--negative_prompt`: Optional text describing what to avoid in the video.
+- `--num_inference_steps`: Number of denoising steps (default: 40).
+- `--seed`: Random seed for reproducibility (default: 42).
+- `--fps`: Frames per second for the saved MP4 (default: 24).
+- `--output`: Path to save the generated video.
+
+## Example materials
+
+??? abstract "text_to_video.py"
+    ``````py
+    --8<-- "examples/offline_inference/ltx2/text_to_video.py"
+    ``````
+
+??? abstract "text_to_video.md"
+    ``````md
+    --8<-- "examples/offline_inference/ltx2/text_to_video.md"
+    ``````