Create a migration guide to migrate from Akka to Pekko

Create a migration guide explaining to a user of Akka how to migrate their project to Pekko.

We hope to automate as much of this as possible under #22 but first we need to determine the steps and lay them out so early adopters can follow them and gain experience.

This should be documented in `docs/src/main/paradox/project/migration-guide-akka-pekko.md`, linked from `migration-guides.md` in the same location, and also linked prominently from the GitHub homepage `README.md` and the project homepage `docs/src/main/paradox/index.md`.

Steps include:
* adapting Akka dependencies `"com.typesafe.akka"` -> `"org.apache.pekko"`
* adapting JAR names `akka-actor_2.13.jar` -> `pekko-actor_2.13.jar`
* adapting imports `import akka.actor` -> `import org.apache.pekko; import pekko.actor` - see #58
* adapting class names `Akka*` to `Pekko*` - see #67
* adapting packages and corresponding paths `akka` -> `org.apache.pekko`, if the codebase inserts code into the Akka namespace
* updating third-party dependencies to versions that depend on Pekko
* adapting configuration to use the `pekko` key - see #63
* discussion of how to approach Akka remote applications - see #108