Unify API Reference Structure

[JonathanWenger]

Problem

With the new Sphinx template, the API Reference has lost some of its navigational structure, making it hard for users to understand the package structure.

Proposed Solution

Across all subpackages of ProbNum, ensure that

• collapsing sidebar navigation exists for at least two levels probnum.subpackage.module
• Solver components are grouped and named correctly
  

ToDo

• create folders in docs/source/api for each subpackage with submodules that should appear in the documentation
• create .rst files with the corresponding automodapi directive