docs: Comply with Diátaxis

[PierreQuinton]

This dev branch makes the transition toward compliance with Diátaxis.

• Changes docs to reference
• Create Explanation
• Create Tutorials
• Create How-to guides

We can partial merge this to main with any release.

When merging main to this, should verify:

• docs/source/docs is empty, if not, it should be moved to docs/source/reference and cross references should be fixed.

[ValerianRey]

Really cool to see that there was some effort to standardize the technical documentation specs.
I still have to read it thoughoughly.

I put the breaking change flag because this will break some existing links (from outside of torchjd to our docs). But I don't think it's a big issue (it's not like there are so many of those links out there).

EDIT: just saw the redirecting script. If I understand correctly the old links will still work. Removing the breaking change flag.

[ValerianRey]

If I understand correctly:

• We have some tutorials but very basic and a bit outdated IMO (https://torchjd.org/stable/examples/basic_usage/, maybe some parts of the readme)
• We have some how-to-guides, which are the rest of our examples. I think those are quite ok, even though we could add more / improve the existing / reorder them.
• We don't really have Explanation apart from the paper and the beginning of the readme. We could probably improve a bit on that.

IMO we should mainly improve on tutorials and simplify the readme (which is currently a mix of everything).

[PierreQuinton]

Exactly right, the goal of this is not to do all of it, just to go in that direction/structure. This definitely will break the ice for newcomers, while keeping experts comfortable. I believe this is a significant improvement. So my goal for now is really to structure it that way, then we shall of course make the content changes.