docs(code-input): clarify `languageAlternatives`, `theme` (#1953)
docs(code-input): clarify `languageAlternatives`, `theme` (#1953)

As it stands, the documentation offers no example of how `languageAlternatives` is structured, which leads many to wrongfully input an array of plain strings. This includes me from the present and the past - I've faced this 3 times already 😆 

I've also linked to the underlying library's site for a reference on themes, as the 4 listed in the current documentation aren't doesn't comprehend all possibilities. Let me know if this makes sense :)
sanity-io/sanity,sanity-io/sanity,sanity-io/sanity,sanity-io/sanity