Automatically resolve dependency versions of Dokka's own plugins via Gradle platform (BOM)

[vmishenev]

If the user has applied Dokka, they should be able to add Dokka's own plugins (such as versioning, mathjax, android, etc) without specifying the version. For example:

dependencies {
    dokkaPlugin("org.jetbrains.dokka:mathjax-plugin")
}

The version should be resolved automatically through Gradle platforms