The top-level examples/ directory held only python/, which made the
examples look Python-only even though Rust, Node and WASM all ship their
own. Add examples/README.md: a single index of every runnable example
across Rust, Python, Node and WASM, each with its run command, plus a note
on the bundled BTCUSDT datasets.
Point the README "Languages" table at the Node backtest example and link
the new index from both the table and the project-layout section.
