Document LTDETRv2 instance segmentation

[yutong-xiang-97]

What has changed and why?

• Split the instance-segmentation documentation into dedicated LTDETRv2 and EoMT pages.
• Document LTDETRv2 instance-segmentation training, inference, model options, transforms, and export (ONNX/TensorRT) workflows.
• Align the EoMT and LTDETRv2 "Model" sections with the panoptic/semantic segmentation docs: unified backbone/weights/license wording, open-by-default model dropdowns, and corrected DINOv2 EoMT backbone names (vit*14).
• Update the documentation landing page, navigation, model matrix, README, and relevant model links to expose the new workflow.

Note: The benchmark plot is still WIP.

How has it been tested?

LightlyTrain documentation.pdf
Instance Segmentation - LightlyTrain documentation.pdf
LTDETRv2 (NEW) - LightlyTrain documentation.pdf
EoMT - LightlyTrain documentation.pdf

Did you update CHANGELOG.md?

• Yes
• Not needed (internal change)

Did you update the documentation?

• Yes
• Not needed (internal change without effects for user)

[yutong-xiang-97]

/review

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