docs: Add a multimodal tutorial

docs/Multimodal Tutorial.md

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+## Getting started
+
+### 1. Find a multimodal model
+
+GGUF models with vision capabilities are uploaded along a `mmproj` file to Hugging Face.
+
+For instance, [unsloth/gemma-3-4b-it-GGUF](https://huggingface.co/unsloth/gemma-3-4b-it-GGUF/tree/main) has this:
+
+<img width="414" height="270" alt="print1" src="https://github.com/user-attachments/assets/ac5aeb61-f6a2-491e-a1f0-47d6e27ea286" />
+
+### 2. Download the model to `user_data/models`
+
+As an example, download
+
+https://huggingface.co/unsloth/gemma-3-4b-it-GGUF/resolve/main/gemma-3-4b-it-Q4_K_S.gguf?download=true
+
+to your `text-generation-webui/user_data/models` folder.
+
+### 3. Download the associated mmproj file to `user_data/mmproj`
+
+Then download
+
+https://huggingface.co/unsloth/gemma-3-4b-it-GGUF/resolve/main/mmproj-F16.gguf?download=true
+
+to your `text-generation-webui/user_data/mmproj` folder. Name it `mmproj-gemma-3-4b-it-F16.gguf` to give it a recognizable name.
+
+### 4. Load the model
+
+1. Launch the web UI
+2. Navigate to the Model tab
+3. Select the GGUF model in the Model dropdown:
+
+<img width="545" height="92" alt="print2" src="https://github.com/user-attachments/assets/3f920f50-e6c3-4768-91e2-20828dd63a1c" />
+
+4. Select the mmproj file in the Multimodal (vision) menu:
+
+<img width="454" height="172" alt="print3" src="https://github.com/user-attachments/assets/a657e20f-0ceb-4d71-9fe4-2b78571d20a6" />
+
+5. Click "Load"
+
+### 5. Send a message with an image
+
+Select your image by clicking on the 📎 icon and send your message:
+
+<img width="368" height="135" alt="print5" src="https://github.com/user-attachments/assets/6175ec9f-04f4-4dba-9382-4ac80d5b0b1f" />
+
+The model will reply with great understanding of the image contents:
+
+<img width="809" height="884" alt="print6" src="https://github.com/user-attachments/assets/be4a8f4d-619d-49e6-86f5-012d89f8db8d" />
+
+## Multimodal with ExLlamaV3
+
+Multimodal also works with the ExLlamaV3 loader (the non-HF one).
+
+No additional files are necessary, just load a multimodal EXL3 model and send an image.
+
+Examples of models that you can use:
+
+- https://huggingface.co/turboderp/gemma-3-27b-it-exl3
+- https://huggingface.co/turboderp/Mistral-Small-3.1-24B-Instruct-2503-exl3
+
+## Multimodal API examples
+
+In the page below you can find some ready-to-use examples:
+
+[Multimodal/vision (llama.cpp and ExLlamaV3)](https://github.com/oobabooga/text-generation-webui/wiki/12-%E2%80%90-OpenAI-API#multimodalvision-llamacpp-and-exllamav3)