doc: add text-based diagram of software architecture in toplevel README.md

[zhouwg]

this PR add text-based diagram of high-level software architecture in toplevel README.md, it will provide a high level comprehension for llama.cpp beginner. from this simple diagram,

• we can clearly see that the ggml machine learning library/framework is a really compact & concise machine learning library/framework!
• we can clearly see what we can do/contribute for this project with our expertise.
• propose PAL(ggml-pal) in toplevel README.md, this difficult and performance-sensitive work can be done by the original authors or other domain experts. this idea comes from my effort on https://github.com/ggml-org/llama.cpp/pull/12049 since 04/2024. in the fact, this is not original idea because this idea can be found at Qualcomm's QNN SDK and Google's Android OS or some commercial massive complicated software in embedded system.