Combobox - update docs to MDX

[jaclynjessup]

📑Documentation

Update the docs for Labs > Combobox using the correct MDX template suggested from #1131 https://github.com/Workday/canvas-kit/blob/master/modules/labs-react/combobox/stories/stories.tsx

AC

The story for Combobox is converted from TSX to MDX and follows one of the 3 prescribed templates