Investigate applying the Diataxis model to the documentation layout

[Zeitsperre]

I find "ALL MODULES" > xclim and "USER API" > API sections redundant.

In the long run, it could be interesting to divide the documentations into categories to group doc pages that belong together. It could be categories such as the ones proposed in diataxis. It's really tedious to do on the existing doc, though.

Originally posted by @bzah in #1308 (review)

[Zeitsperre]

To do: Divide this into sub-categories.

[coxipi]

So do we want to apply this approach to notebooks too? I would say that current notebooks are currently much more oriented towards "practical steps". But perhaps this comes quite naturally with the notebook format.