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/stream/io/StreamFileDocTest.java

@@ -104,7 +104,7 @@ public class StreamFileDocTest extends AbstractJavaTest {
     try {
       // #file-sink
       Sink<ByteString, CompletionStage<IOResult>> fileSink = FileIO.toPath(file);
-      Source<String, NotUsed> textSource = Source.single("Hello Akka Stream!");
+      Source<String, NotUsed> textSource = Source.single("Hello Pekko Stream!");
 
       CompletionStage<IOResult> ioResult =
           textSource.map(ByteString::fromString).runWith(fileSink, system);