Introduce some hierarchy in the API

[Marius1311]

... At the moment, our API menu looks like this:

That's not very nice, the list is way too long. In contrast, scVi-tools displays this much nicer with some hierarchical structure:

I suggest to do the same for CellRank, let's use the sub-headings we already have, e.g. Kernels, External Kernels, Estimators, Plotting, Models, Datasets. This will look much cleaner.

[Marius1311]

BTW, @michalk8, what did you plan to put under the developer API? Can we also move this as an extra sub-heading, like the other ones above? Or would you like to keep a top-level heading for this?

[michalk8]

BTW, @michalk8, what did you plan to put under the developer API?

What was there named before as Classes.

Or would you like to keep a top-level heading for this?

No preference.

[michalk8]

That's not very nice, the list is way too long. In contrast, scVi-tools displays this much nicer with some hierarchical structure:

They also have flat list (it's because the theme), just expand API. Not sure how to change this atm.

[Marius1311]

But they do have categories, like User and Develper, can't we do the same for the categories we have (like kernels and estimators)?