Setup pekko-platform-guide to build with pekko-sbt-paradox

Relates to https://github.com/apache/incubator-pekko/issues/244

See discussion on the mailing list:

```

PJ Fanning:
Any chance we could make the doc build use the pekko-sbt-paradox plugin like we use elsewhere?

Sambyng:
I think this is probably the ideal long term situation. I would say that we should do this now too.

I assume this will require the following - anything else?

- Docs conversion from asciidoc to markdown (potential bugs / features lost introduced during conversion)
Seems like there's a CLI tool for this here: https://github.com/opendevise/downdoc

- Update of codebase with a build.sbt file + paradox setup code / config etc.

- Update to use the pekko guide UI instead of the lightbend UI

- Strip out the current antora / asciidoctor code.

- Update the manual publishing guide: https://github.com/apache/incubator-pekko-site/wiki/Manual-Publishing

Pros: Consistent approach, Easily get the Apache Pekko UI in, Using a build tool that we control.
Cons: More work to get platform guide out. Bugs converting docs that will need ironing out.
```