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

[Jolanrensen]

Ever since the doc processors was updated to 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.

[Jolanrensen]

Let's introduce this gradually when we make new doc interfaces or we spot a good place for it