wasm : add note about worker.js file generation [no ci] (#3133)

wasm : add note about worker.js file generation [no ci] (#3133)

This commit updates the documentation for the WASM examples to include a
note about the generation of the `worker.js` file. As of Emscripten
3.1.58 (April 2024), separate worker.js files are no longer generated
and the worker is embedded in the main JS file.

The motivation for this change is to inform users about the new behavior
of Emscripten and why the `worker.js` file may not be present.

Refs: https://github.com/ggml-org/whisper.cpp/issues/3123