rewrite pekko-platform-guide to use pekko artifacts and to remove Akka refs

https://github.com/apache/incubator-pekko-platform-guide

This guide is referenced quite a bit from the main docs associated with this repo.

Edit: the doc refs from this repo have been temporarily removed which means that updating the pekko-platform-guide is not as urgent. See https://github.com/apache/incubator-pekko/commit/658e4bc4b0b9fd0305f435ab02c445a5344783aa

If and when the pekko-platform-guide is updated and published, we could revert that commit and tidy up the links to ensue they point to the right place (where the new pekko-platform-guide docs are published).