[focusgroup] Fix explainer demos so the correct roles apply

[lukewarlow]

There's at least two demos in the explainer that have incorrect roles:

1. The nested example doesn't set role="none" on the <li> so they still have listitem roles. (Shout out to Adrian for spotting that https://adrianroselli.com/2026/07/focusgroup-tests.html#Opinions )

2. The usage of <ul focusgroup=...> leads to those demos keeping a container role of list rather than for example "menubar".

I'm not sure if the ul, and li usages are neccessary in these demos (I would think not given the bugs) but the demos should be fixed up.

[lukewarlow]

cc @janewman