fix: Improve inventory object types (incorrectly called roles in mkdocstrings)

[pawamoy]

Issue-339: #339

For reviewers

• I did not use AI
• I used AI and thoroughly reviewed every code/docs change

Description of the change

See #339, Sphinx inventories do not contain "roles" but "object types".

[pawamoy]

The property one was easy enough, although I'd prefer to use an hypothetical is_property attribute in the Griffe object. The exception one will have to wait as there's no easy way to check that a class inherits (directly or indirectly) from Exception or BaseException. This will need an update in Griffe (adding for example an is_exception property on class objects). The method one was trivial.

I didn't add support for classmethod and staticmethod, seeing how little they are used in the wild anyway.

Finally I kept parameter and typeparameter, although they are invalid, since some Sphinx extensions already take the liberty to use parameter. We're just a bit more consistent now. If this really is problematic (do some inventory loaders enforce valid types?), we can use obj instead, I don't mind.