Update documentation to reflect changes in package design

[ChristianGeng]

Title: Improve API Documentation Structure

Description:

As a follow-up to the redesign of the package in #246, I noticed that the high-level description of the package's architecture is not prominently featured in the API documentation. Specifically, the explanation of the separation between the backend (audbackend.backend) and interface (audbackend.interface) classes is currently located at the end of the API doc.

To improve the readability and usability of the documentation, I propose refactoring the documentation to move this high-level design description to a more prominent location, such as:

• the top of the Developer Guide
• the beginning of the Usage Guide

This will help readers quickly understand the overall architecture and design of the package, making it easier for them to navigate the documentation and get started with using the API.

Task:

Refactor the API documentation to move the high-level design description to a more prominent location.

Related Issue: #246