Improve our documentation

[Marius1311]

We can improve our documentation in a few places to make it easier to get started. Here are some ideas:

• Improve high-level package description on the landing page.
• Include key applications section on landing page
• Include a "getting started" section on the landing page, which points to our user guide, examples and tutorials, and explains what each of these is for.
• Introduce some hierarchy in the user API, so that we have collapsable sections in the menu on the left (similar to e.g. scvi: https://docs.scvi-tools.org/en/stable/index.html). That would make it easier to find problems, datsaets, etc. This hierachyt actually exists already, it just isn't reflected in the menu on the left. #773
• Exchange the large logo on the landing page with a concept figure that gives a high-level overview of the package. The moscot logo is included anyways in the top left corner, so this is just a duplication.

[MUCDK]

Thanks @Marius1311 !

Regarding

1. would you suggest high-level description wrt OT or rather which tasks can be performed?
2. what are key applications? each of the bio problems? or a subset thereof?
3. would you then replace the boxes? Or have it in addition? Or have it integrated into the current structure?
4. I think that makes sense, maybe @LeonStadelmann you can start doing this? I.e. for every module we would like to be able to collapse the corresponding submodules. Atm, everything is displayed.
5. makes sense, would you use Fig. 1 or something else?

[Marius1311]

HI @MUCDK, please see #774 where I made a few suggestions!