Dead tradeoffs link in FAQ (Write tradeoffs.md)

__Expected behavior:__
Clicking "Check out [tradeoffs|https://github.com/cssinjs/jss/blob/master/docs/tradeoffs.md] for more information." opens filled tradeoffs page.

__Describe the bug:__
Instead I get a 404.

__Reproduction:__
Navigate to: https://cssinjs.org/faq
Under item "6. Does JSS has overhead?"
Click: "Check out [tradeoffs|https://github.com/cssinjs/jss/blob/master/docs/tradeoffs.md] for more information."

__Related to:__
Issue New docs #932 "Write tradeoffs.md" from 2018.

(This issue will hopefully prevent others from having to search)

Inspect the FAQ entry for “6. Does JSS has overhead?” and the related “New docs #932” issue, then review the expected docs location at docs/tradeoffs.md. Fill that page with the tradeoffs information and verify the FAQ link opens it instead of returning a 404.

Written by the indexing model from the issue text.