Update LLM documents and websites

[mergennachin]

Current status quo:

ET Website:
https://docs.pytorch.org/executorch/stable/llm/llama.html
https://docs.pytorch.org/executorch/stable/llm/llama-demo-android.html
https://docs.pytorch.org/executorch/stable/llm/llama-demo-ios.html
https://docs.pytorch.org/executorch/stable/llm/build-run-llama3-qualcomm-ai-engine-direct-backend.html
https://docs.pytorch.org/executorch/stable/llm/getting-started.html

ET Readmes:
LLama
Qwen
Phi-4
Llava

Proposal:

AOT flow:

• Documentation about export_llm
• For each models, how it is used in examples page.

Runtime flow:

• How to use C++, Swift and Java APIs in extensions/llm
• Building examples app, not only Llama but also other models like Qwen3, Phi4-mini.

[mergennachin]

#12757