Update document memory_tune.md: fix embed wamr link error, fix description error of wasm operand stack size, update picture. Fix wasm_runtime_call_wasm_a issue reported by #1003 and update sample basic to call this API.
Update README.md, add "Getting Started", “Performance and Footprint”, ”Use Cases", and refine some sections. Add memory tune document and fix wasm-c-api document.