docs: link to the SSWG's `perf` guide (#1882)
docs: link to the SSWG's `perf` guide (#1882)

Motivation:

Instead of reproducing partial information, it makes more sense to link to the SSWG's guide which is more complete.

Modification:

Remove incomplete information and swap for a link.

Result:

Better docs.

Co-authored-by: Cory Benfield <e2b4f0650e369574e9b0029c3b3893634b6d4097@apple.com>
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio