Remove readthedocs with sbt-microsite multi version support

[muuki88]

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The sbt microsite plugin now has a proper multi version support. This would solve the following issues

• dependency on readthedocs.io which cause a few issues in the past (tags weren't built)
• [https://www.scala-sbt.org/sbt-native-packager/] would have all versions not just the latest
• no permission problems if someone has no access to readthedocs.io

Also the project has a more "scala like" look as quite a few projects use this plugin.

Steps

• Rewrite docs from rst to markdown
• Integration plugin
• Define a way on how to manage the micrositeVersionList (all, last x tags, at least one for each major version?)