Document workaround for `'emscripten' is not a valid Platform`

[jtpio]

Problem

As noticed in https://github.com/jupyterlite/jupyterlite-sphinx/issues/177 and various linked issues and PRs, using the latest jupyterlite-xeus release may lead to the following error during a jupyter lite build:

'emscripten' is not a valid Platform

Suggested Improvement

The fix is available is jupyterlite>=0.2.0a0, and requires using micromamba in the build environment: https://github.com/jupyterlite/xeus/pull/88