Add xref to emscripten wheel building instructions to platform mismatch error message

[hoodmane]

Add the following language

See https://pyodide.org/en/stable/development/building-and-testing-packages.html for info on how to build an emscripten wheel

To the error message here: https://github.com/pyodide/micropip/blob/main/micropip/_micropip.py#L118