Hide backend in API documentation

[hagenw]

In #167 we introduce a new abstract interface layer between backends and the user. Which means a normal user will need to look only at the interface API. For that reason we should hide parts of the backend API from the user, e.g. all the methods that are inherited from audbackend.Backend in audbackend.Artifactory and audbackend.FileSystem and instead add a link to the interface section.

It might still be of interest to show the API for audbackend.Backend as this is needed for developer who want to create a new interface.

[hagenw]

I will start working on this one.

[hagenw]

Changes to the documentation will be easier after we changed the package structure as proposed in #170. Will pause until that has happened.

[frankenjoe]

@hagenw are you working on #170? Otherwise I can also do it.

[hagenw]

No, I'm not working on audbackend at the moment, feel free to start with #170.

[frankenjoe]

Now with the separation of backend and interface, I'm not completely convinced if we still have to hide parts of audbackend.backend.* in the API. Maybe it's sufficient if we add a note that he sub-module is only relevant for developers?

[hagenw]

I agree. I continued with #171, but I simply change the usage documentation by splitting it up into a section for users and one for developers.