Missing redirects for /setup/

[galeyang]

There is no redirects for this URL: https://flutter.io/setup/

FYI: one of the entry points is on API docs' landing page --- install (https://flutter.io/setup/)

[kwalrath]

/setup/ is the top 404, by far.

[chalin]

Closed by #1722. Thanks again Kathy!

(For the record, /setup/ wasn't in the list of URLs I was given as a base to create the redirect rules from.)