cases: extract how-to/tutorial parts into separate pages

[jorgeorpinel]

Extracted from #1716 (comment) +

In the case of any how-tos, into the new How-to section (see #899);
In the case of tutorials, into subpages like https://dvc.org/doc/use-cases/versioning-data-and-model-files/tutorial (for now).

See https://documentation.divio.com/introduction/ for useful guidelines.

[jorgeorpinel]

I guess we're on track to address this now and it's not very well defined so I'll just close it now.