Implement docs generation in "Kotlin as Java" format, extract Dokka version to properties

[kamildoleglo]

• The pluginManagement block is debatable, however that's probably the easiest way to ensure the version compatibility between Dokka and Dokka's plugins (a slightly worse way would be to use a buildscript block)
• The -modules.html renaming task probably won't be needed after 1.4.20.2 version
• Unfortunately kotlinAsJava format crashes for the :zircon.core subproject, please observe https://github.com/Kotlin/dokka/issues/1663 for a fix

[adam-arold]

@kamildoleglo this is great! 👍