Point docs to Swift Package Index (#416)

Motivation:

The NIO docs are now published on the Swift Package Index but the README
still refers to GitHub pages.

Modifications:

- Update README and other docs to point to Swift Package Index.

Result:

Documentation links work

Author: glbrntt

README.md

@@ -4,7 +4,7 @@ SwiftNIO SSL is a Swift package that contains an implementation of TLS based on
 
 The name is inspired primarily by the names of the library this package uses (BoringSSL), and not because we don't know the name of the protocol. We know the protocol is TLS!
 
-To get started, check out the [API docs](https://apple.github.io/swift-nio-ssl/docs/current/NIOSSL/index.html).
+To get started, check out the [API docs](https://swiftpackageindex.com/apple/swift-nio-ssl/main/documentation/niossl).
 
 ## Using SwiftNIO SSL