Fix clashing imports in docs

When I build the docs, autodoc tries to import things from inside doc/ase which is not a real python package.

This MR fixes the issue by renaming doc/ase to doc/modules which was the first thing I could think of.

Do we want this fix or a similar one? Or maybe my computer has a bad setup and we shouldn't fix it at all?

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.