change docs to use pekko paradox plugin

* see https://github.com/apache/incubator-pekko-connectors-samples/blob/main/docs/build.sbt
* have a look at https://github.com/apache/incubator-pekko to see how it sets up the PekkoParadoxPlugin

Start with docs/build.sbt in the pekko-connectors-samples repository, then compare its setup with docs/build.sbt in incubator-pekko-connectors-samples and the Pekko repository setup. Confirm how PekkoParadoxPlugin is configured and update the sample documentation build so it uses that plugin; done means the documentation build works with the new setup.

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