Annotate non-user facing documentation interfaces with `@ExcludeFromSources`

Ever since the doc processors was updated to [v0.3.2](https://github.com/Jolanrensen/docProcessorGradlePlugin/releases/tag/v0.3.2), we've had the `@ExcludeFromSources` annotation at our disposal.
This allows temporary doc-interfaces to be excluded from our preprocessed source files, cleaning them up a lot. It might also shrink our bytecode size, since there are fewer interfaces to be compiled.
Finally, it could help in other future applications. For instance, the `dokkaJekyll` task failed before I added `@ExcludeFromSources` to `api/all.kt`.