Default docs to `stable`

[jtpio]

Problem

Currently https://jupyterlite-xeus.readthedocs.io redirects to https://jupyterlite-xeus.readthedocs.io/en/latest/ by default.

And the docs mention the use of the jupyterlite-xeus stable with changes from the pre-release, which can be confusing to users.

Suggested Improvement

Default the docs to https://jupyterlite-xeus.readthedocs.io/en/stable.

[jtpio]

Additionally it would be useful to enable the version switcher, like for the JupyterLite docs: