docs: rewrite README.md as clean entry point for all language packages

Shrink README from 1343 lines to 210 lines. Remove raw WASM examples
(Chicory, wasmtime-py, Node.js) that are now superseded by wrapper
packages. Remove detailed API docs, memory model, and architecture
content that already lives in ARCHITECTURE.md, HOST_FUNCTIONS.md, and
per-language READMEs.

New structure: language package table, concise quick-start examples for
all 6 languages (Java, Go, .NET, Python, JS, Rust), "How It Works"
section explaining the 3 host-side optimizations, trimmed custom
operators reference, and documentation link table.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: aepfli