Replaced doc.akka.io with pekko.apache.org (#120)

* modified all references in docs module

* replaced in inline scala docs

* renamed variable and changed access specifier

* reanmed in remaining places

* Adjusted path (instead of `akka` used `pekko`)

* Where applicable akka.io is replaced with pekko.apache.org

* replaced akka with pekko in the path

* Removed TODO

* Removed migration guide to gRPC (there is no user using pekko at the moment so no need of migration guide)

* Added back migration to gRPC section

* removed TODO

* alpakka is replaced with pekko-connectors

* the Akka online documentation -> the Pekko online documentation

* referring akka security docs

* reffering to akka migration guides (pekko is not managing them)

* Using pekkoBaseURL for platform-guide.base_url

* removed unnecessary redirects and used current instead of 2.6

* Changed akka discovery to pekko discovery

* Some more replacement

* fix merge issue

* revert platform-guide change because we

---------

Co-authored-by: PJ Fanning <pjfanning@users.noreply.github.com>

actor-testkit-typed/src/main/scala/org/apache/pekko/actor/testkit/typed/internal/StubbedActorContext.scala

@@ -55,7 +55,7 @@ private[pekko] final class FunctionRef[-T](override val path: ActorPath, send: (
   override def provider: ActorRefProvider =
     throw new UnsupportedOperationException(
       "ActorRefs created for synchronous testing cannot be used as targets for asking. Use asynchronous testing instead. " +
-      "See https://doc.akka.io/docs/akka/current/typed/testing.html#asynchronous-testing")
+      "See https://pekko.apache.org/docs/pekko/current/typed/testing.html#asynchronous-testing")
 
   // impl InternalRecipientRef
   def isTerminated: Boolean = false