DOC: restore toctree maxdepth
Closes https://github.com/pandas-dev/pandas/issues/25131

I used a `maxdepth` of 3 now, instead of the original 4.

Now, the problem is that this makes the toctree on the main page (the one in the actual page body, not the one in the left sidebar) again very long and not very usable ..

cc @datapythonista @neili02