Update the manuals/documentation

[LHBO]

In this PR, we fix several issues in the documentation:

1. Fixed such that internal no longer appears as an ... argument to explain() and explain_forecast().
2. Fixed so that the method-specific arguments in the ... documentation are in alphabetical order in explain(), explain_forecast() and explain_tripledot_docs(). The approaches are not alphabetically ordered in approaches, but that is okay. There, it makes sense to maybe put the best/ones we recommend first. Discuss with Martin.
3. Fixed an error in empirical.eta that removed half of a sentence from the parameter description.
4. Fixed that vaeac_get_data_objects() was exported but should have been an internal function.
5. I have fixed finalize_explanation() so that it no longer includes documentation from explain(), which should not be there. I have also added my name as a co-author, as I have the MSEv thing at the end of the function.
6. I fixed the prepare_data() documentation, which stated that internal was not used, which it is. Also, I added Martin's name as a co-author. Mine was the only name since I included my name in prepare_data.vaeac().
7. Fixed imports by removing the imports that are no longer used. Also, note that several more can be removed if one uses, e.g., data.table::copy instead of just copy. Currently, there is no consistency across methods when to include the library prefix and not. Discuss with @martinju. Also, sourceCpp is no longer needed, but removing it will cause the checks to give a lot of warnings. (This is PR Update the imports #377)
8. Added plot_SV_several_approaches and plot_MSEv_eval_crit as main functions in the main vignette.
9. Added regression_separate and regression_surrogate in get_factor_approaches(), as they were missing and updated the corresponding snaps.