Improve documentation about return types for the API

[thorbjoernl]

It is currently not clear what return type is returned by the API.

• It should be made clear that AccessType.OBJ results in a dict (or dict-like) object. It looks like using overloads may help make the typehints clearer, need to investigate: https://stackoverflow.com/questions/52445559/how-can-i-type-hint-a-function-where-the-return-type-depends-on-the-input-type-o
• https://peps.python.org/pep-0586/#literals-enums-and-forward-references
• F12ing an aerovaldb method currently takes you to the base class which is of limited usefullness when investigating. See if there is a better way or add a reference in the documentation.

Docs formatting issues:

• https://aerovaldb.readthedocs.io/en/latest/api.html#aerovaldb.aerovaldb.AerovalDB.put_map_overlay

[thorbjoernl]

@lewisblake FYI