docs: added `additional_resources` folder (#4748)
# docs: added `additional_resources` folder
The additional resource files were inside the doc top-level folder,
which polluted the top-level folder.
- added the `additional_resources` folder and moved correspondent files
to this folder;
- fixed a broken link to the "Model comparison" page (model_laboratory
notebook)
- fixed a broken link to one of the YouTube videos (sorry, it is not
directly related to this PR)
## Who can review?
@dev2049
