Exclude generated outer classes from including into documentation

[alexander-yevsyukov]

Currently, outer classes generated by Protobuf are included into Javadocs and KDocs. For example, in io.spine.protodata package we have AstProto, DeclarationsProto, FileProto, InsertionProto, and other classes suffixes with Proto for all Protobuf source code files. There's nothing useful for end users in these documents, they just add up noice by appearing in the list of useful items.

We need to exclude these classes from generated documentation in both Javadoc and KDoc.

One of the possible approaches would be to add @Internal annotation for the generated outer classes.
Alternatively, we can add a custom annotation similar to @suppress in KDoc and handle it.

[armiol]

If we go for the approach based on @Internal annotation, the repository dokka-tools should be brought to life first. Currently, it's a bit outdated.

Alternatively (and I would NOT go for it right now), there is a way to introduce a custom Javadoc tag, similar to KDoc's @hide. And create a Dokka plugin which would handle it, similar how it handles @hide for Android.