chore(docs): add opencollective recommended refund disclaimer to /sponsors (#12575)

Author: ndom91

docs/pages/sponsors.mdx

@@ -181,3 +181,5 @@ Auth.js is a set of libraries that make it easy to add authentication to any app
 </div>
 
 If you or your company enjoys Auth.js, please consider [sponsoring via OpenCollective](https://opencollective.com/nextauth) too. Choose a higher tier to appear on this page. Sponsors are listed in alphabetical order in their respective group.
+
+> This is a donation. No goods or services are expected in return. Any requests for refunds for those purposes will be rejected.