docs: rewrite README.md as clean entry point (#109)

## Summary

- Shrinks README from **1,343 lines to 210 lines** (84% reduction)
- Removes raw WASM examples (Chicory, wasmtime-py, Node.js) superseded
by wrapper packages
- Removes detailed API docs, memory model, and architecture content that
already lives in ARCHITECTURE.md, HOST_FUNCTIONS.md, and per-language
READMEs
- Adds language package table covering all 6 languages (Java, Go, .NET,
Python, JS, Rust)
- Adds concise quick-start examples for every language (5-10 lines each)
- Adds "How It Works" section explaining the 3 host-side optimizations
- Adds documentation link table pointing to all deep-dive references

## Test plan

- [x] All internal links verified (ARCHITECTURE.md, BENCHMARKS.md,
HOST_FUNCTIONS.md, CONTRIBUTING.md, java/README.md, python/README.md,
LICENSE)
- [ ] Confirm README renders correctly on GitHub
- [ ] Verify no broken markdown (tables, code blocks, links)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: aepfli