Add dokkaPlugin configuration to example code

[covercash2]

In the process of trying to apply a dokkaPlugin, it was in example code to include this line but not mentioned in the documentation. This is a one line change to add that line to the Kotlin script example. I'm not certain if this also applies to the Maven and Groovy implementations, but I wanted to at least start the conversation

[whyoleg]

Hey!
Sorry for the long delay with review.
In most of the cases the dokkaPlugin accessor for dependencies should be auto-generated by Gradle based on configuration created by Dokka Gradle Plugin.
F.e like in dokka-kotlinAsJava-example we don't need to declare dokkaPlugin.
Still, there are some cases, when Gradle will not generate this accessor. One of such cases is when Dokka Gradle Plugin is not applied to current project (and you configure only subprojects) or when it's applied not via plugins block.
This case is shown in dokka-versioning-multimodule-example.
Overall, current Gradle guidelines are suggesting not to use such approach for configuration. But still, there are other cases when dokkaPlugin accessor is not generated, so may be it will be good to add a note in documentation, something like:

• If dokkaPlugin is shown red/unavailable it's possible to add val dokkaPlugin by configurations before dependencies block as Gradle failed to generate accessor in this case.

Regarding Groovy: there should be no such issues, as there all accessors are resolved dynamically.
Regarding Maven: there should be no such issues, as the configuration there is different from Gradle.