Renames in doc #98 (#100)

* Rename some base_url values in Paradox.scala
* Rename Akka to Pekko in documentation, also adapt/remove small blocks of text that were no longer consistent or adequate
* Rename Akka to Pekko in some related files outside the doc directory
* Rename akka-bom to pekko-bom
* Rename artifacts prefixes from akka to pekko
* Add links to Apache documents in licenses.md
* Add links to Akka migration guides for earlier versions

docs/src/test/java/jdocs/dispatcher/MyUnboundedMailbox.java

@@ -56,7 +56,7 @@ public class MyUnboundedMailbox
     }
   }
 
-  // This constructor signature must exist, it will be called by Akka
+  // This constructor signature must exist, it will be called by Pekko
   public MyUnboundedMailbox(ActorSystem.Settings settings, Config config) {
     // put your initialization code here
   }