[sil_ipa] no charts for mobile keyboard

[mcdurdin]

Today I assisted a user trying to find ˀ on their mobile keyboard -- and it took us some time to find it under q; it makes sense but wasn't obvious (no separate question mark key which was what we looked for first). I checked the help docs and realised that we have no documentation on the mappings, so this might be something to consider.

Note: I want to improve the mobile keyboard automatic documentation to have it generate charts of all long-press keys which would simplify this issue sigificantly, so treat this as low-priority, just documenting the issue.