Restructure ET documentation with 'Platform First' model

Author: psiddh

docs/source/android-section.md

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+# Android
+
+## Quick Start & Integration
+[Getting started with ExecuTorch on Android](using-executorch-android)
+
+## Available Backends
+
+### CPU Backends
+- [XNNPACK Backend](backends-xnnpack) - Optimized CPU inference
+
+### GPU Backends
+- [Vulkan Backend](backends-vulkan) - Cross-platform GPU acceleration
+
+### NPU/Accelerator Backends
+- [Qualcomm AI Engine Backend](backends-qualcomm) - Snapdragon NPU acceleration
+- [MediaTek Backend](backends-mediatek) - MediaTek NPU support
+- [Samsung Exynos Backend](backends-samsung-exynos) - Samsung NPU acceleration
+
+## Working with LLMs - Android Examples
+
+For comprehensive Android LLM examples and tutorials:
+
+**[→ Android LLM Examples Repository](https://github.com/meta-pytorch/executorch-examples/tree/main/llm/android)**
+
+This repository contains:
+- Complete Android LLM integration examples
+- Step-by-step tutorials
+- Sample applications
+- Performance benchmarks
+
+## Demo Applications
+
+Explore our collection of Android demo applications:
+
+**[→ DeepLabV3 Demo App](https://github.com/meta-pytorch/executorch-examples/tree/main/dl3/android/DeepLabV3Demo#executorch-android-demo-app)**
+
+Additional demo applications available:
+- Computer Vision demos
+- Natural Language Processing examples
+- Audio processing applications
+
+## Troubleshooting
+
+For common Android issues and solutions, see our [Troubleshooting Guide](using-executorch-troubleshooting).
+
+```{toctree}
+:maxdepth: 1
+:hidden:
+
+using-executorch-android
+backends-xnnpack
+backends-vulkan
+backends-qualcomm
+backends-mediatek
+backends-samsung-exynos
+using-executorch-troubleshooting
+```

docs/source/backends-samsung-exynos.md

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+# Samsung Exynos Backend (TBD)

docs/source/desktop-section.md

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+# Desktop & Laptop
+
+## Platform Overview
+[Desktop & Laptop Platform Overview](platforms-desktop)
+
+## C++ Runtime Integration
+[C++ Runtime Integration Guide](using-executorch-cpp)
+
+## Linux
+### Available Backends
+- [XNNPACK Backend](backends-xnnpack) - CPU optimization
+- [Vulkan Backend](backends-vulkan) - GPU acceleration
+- [OpenVINO Backend](build-run-openvino) - Intel hardware optimization
+- [ARM Ethos-U Backend](backends-arm-ethos-u) - ARM64 NPU support
+
+## macOS
+### Available Backends
+- [CoreML Backend](backends-coreml) - recommended for Apple hardware
+- [MPS Backend](backends-mps) - Apple Silicon GPU acceleration
+- [XNNPACK Backend](backends-xnnpack) - CPU optimization
+- [Vulkan Backend](backends-vulkan) - GPU acceleration
+
+## Windows
+### Available Backends
+- [XNNPACK Backend](backends-xnnpack) - CPU optimization
+- [Vulkan Backend](backends-vulkan) - GPU acceleration
+- [OpenVINO Backend](build-run-openvino) - Intel hardware optimization
+
+```{toctree}
+:maxdepth: 1
+:hidden:
+
+platforms-desktop
+using-executorch-cpp
+backends-xnnpack
+backends-vulkan
+build-run-openvino
+backends-arm-ethos-u
+backends-coreml
+backends-mps
+```

docs/source/embedded-section.md

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+# Embedded
+
+## Platform Overview
+[Embedded Platform Overview](platforms-embedded)
+
+## C++ Runtime Integration
+[C++ Runtime Integration Guide](using-executorch-cpp)
+
+## Microcontrollers
+- [Cadence Xtensa Backend](backends-cadence) - Xtensa processor support
+- [ARM Ethos-U NPU Backend](backends-arm-ethos-u) - ARM microcontroller NPU
+- [NXP eIQ Neutron Backend](backend-nxp) - NXP hardware optimization
+
+## Custom Hardware Integration
+[Custom Backend Development Guide](backend-delegates-integration)
+
+## WebAssembly (Coming Soon)
+[WebAssembly Support](using-executorch-cpp#webassembly)
+
+```{toctree}
+:maxdepth: 1
+:hidden:
+
+platforms-embedded
+using-executorch-cpp
+backends-cadence
+backends-arm-ethos-u
+backend-nxp
+backend-delegates-integration
+```

docs/source/fundamentals.md

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+# Fundamentals
+
+## Model Export & Lowering
+[Learn more](using-executorch-export)
+
+## Runtime Architecture
+[Learn more](runtime-overview)
+
+## Memory Management
+[Learn more](compiler-memory-planning)
+
+## Performance Concepts
+[Learn more](runtime-profiling)