docs: ui.picker (#705)

Closes #685 

**Additional Info:**
- Did not include any of the sections containing dynamic items, since
our picker does not support the `items` prop
- Did not include section with links, since we do not have a `ui.link`
- Did not include section with avatar, since we do not currently have a
`ui.avatar`
- Changed "Asynchronous Loading Section" to "Loading", since we do not
have a `on_load_more` prop exposed