chore: Revert "chore: Revert "change: use published Keyman Engine API to fetch keyboards in keyboard help pages""

[mcdurdin]

Reverts: keymanapp/help.keyman.com#1518 Fixes: keymanapp/keyman#12345 Co-authored-by: @ermshiperete

Alongside additional CSS fixes:

1. Remove unnecessary font-size hammers for the OSK as KMW does a good job of managing font size itself now
2. Remove unnecessary z-index: -2 for OSK that blocked interaction with earlier versions of KMW, as KMW documentation keyboards are no longer interactive

[ermshiperete]

Dene keyboard:

Coptic (Greek) keyboard:

Vai (Athinkra) keyboard: